What Does Sleep Do?

Sleep is something that we all do, or at least we all have done. Despite this, it is still a somewhat mysterious thing. You would think that for something we do for one-third of our lives, we would have a pretty damn good understanding of it. However, this is not really the case. If you asked any random person on the street about sleep, they would likely give you any number of buzzwords related to sleep, but if you probe further and ask about why we need to sleep, how sleep is regulated, what sleep hygiene is or how to get a good night’s sleep, you will likely receive more myths and logical inconsistencies than if you were to talk about something like training or nutrition.

Sleep is “just something we do”, that is until, of course, something goes wrong, and you just can’t sleep. All of a sudden, sleep isn’t something you just do, it is the exact opposite. You toss and turn, and lay there, but that mysterious ether of sleep never engulfs you. So, in this article, we are going to explore what sleep does in considerable depth, helping you understand why this foundation matters so profoundly for living well.

What Does Sleep Do? Unravelling The Mystery of Sleep

We know an awful lot about sleep, but we don’t know everything. Sleep is a very complex phenomenon, and it also crosses many fields of science, which makes it quite difficult to get a comprehensive picture of sleep. It is often discussed as a brain phenomenon, mainly because that is where we have been looking for answers, but it is actually a total body phenomenon. Some animals without a brain (such as jellyfish) do actually sleep, and some animals with a brain (such as tuna) don’t sleep. Some animals (such as dolphins) have very strange sleep patterns, with only half of their brain “asleep” during their sleep, alternating to get “full” rest.

Only looking at the brain doesn’t give us all the information either, as sleep is actually a full-body phenomenon and many things happen across the body that aren’t measurable if we only look at the brain. Sleep permeates everything in our body, and involves and has effects across many body systems. However, having said all of that, the brain is still heavily involved in sleep and understanding what is going on here will allow us to expand the conversation to other areas of the body.

Covering all of what sleep does is simply not going to be possible in this article. However, we don’t need to know it all to appreciate why sleep matters. The main goal of this article is to give you a comprehensive understanding of what sleep does and why it serves as an irreplaceable foundation for human flourishing. This isn’t about making you a sleep scientist; it’s about helping you understand why sleep isn’t something you can afford to neglect if you care about living fully.

At a top-level view, the basic effects of sleep seem to be repair, recovery, consolidation and “setting things up” for the future. I always use the analogy of the body (including the brain) being a factory. Throughout the day, the body is building things and breaking other things down, and in doing all of that, there is some wear and tear on the machinery, along with some waste being created. During the night shift (sleep), the factory is cleaned out, any wear and tear on the machines is repaired, and the management sits down and consolidates all of the information that has been accumulated that day and tries to plan for the future.

I like this analogy because it is easy to understand, and it also gets you thinking that sleep is actually an active process and not just a passive process. The night shift of the body factory actually involves work being done, rather than nothing occurring at all. It requires energy, and if you want to set your factory up well for the future, you need to actually put in some work to ensure the night shift is able to do everything they need to do to have the next day be a productive one. What we are trying to do with our sleep practices is set things up so the night shift has the best chance of doing everything they need to do.

This metaphor becomes more tangible once we explore what sleep does in greater depth and examine how sleep affects different parts of the body and different body systems. What really helps us understand what sleep does is to see what happens when sleep isn’t in a good place. However, the things that break first when sleep is poor aren’t necessarily representative of all the things that sleep does. But it does give us something to work from. Covering a combination of what goes wrong when sleep isn’t good and then noting the effects sleep has on different body processes should give you a comprehensive picture of what sleep does.

We’re going to focus on health, nutrition, exercise and stress, as they are the most relevant to living well. But sleep does impact other areas (such as productivity, financial health, and relationships) that also feed back onto your overall well-being. Your financial health influences your actual health, and sleep impacts your ability to earn money. Your relationships affect your stress levels, and sleep influences how you show up for the people you care about. Everything is connected.

What Does Sleep Do for Your Physical Health?

It is quite hard to convey how good sleep affects the body because it kind of just happens in the background, and it almost feels like it isn’t really impacting our health when sleep is good. It is easier to see the impact sleep has on your overall health by discussing what goes wrong when sleep isn’t in a good place. However, it does bear noting that “bad sleep” is a very broad category. Bad sleep could be the occasional hour missed here and there, or it could be chronically missing hours every night, or it could be missing a full night’s sleep (in)frequently, and so on. The health issues that crop up with each of these different situations is likely to be different, but we can still get a general picture of what is going on.

I am going to focus the discussion on the effects of chronically missing a few hours of sleep, as that seems to be the most consistent issue people encounter. They aren’t getting enough sleep most nights, and maybe they’re getting a bit more on the weekends. This is the pattern that affects most people, and understanding what happens in this scenario will help you see why sleep deserves your attention.

Before we examine the effects sleep has on different systems and processes, I want to note that sleep really does have its fingers in every pie when it comes to health. As it is implicated in repair and recovery, you can see how that would be something that is pretty important to keeping you healthy. This really does run the gamut from helping you heal little cuts to healing from major injuries, and also repairing the general wear and tear that occurs in daily life for all your body tissues, including your internal organs.

Good quality sleep is also vital for the adaptation process. The body doesn’t only repair to where you previously were, it also builds a bit more to help ensure that you are more resilient to the stressors you are likely to encounter in the future. So poor sleep potentially not only reduces your ability to repair and recover to baseline, but it also potentially reduces your ability to adapt and become more resilient. Good quality sleep is a bit of a panacea, and whilst the analogy is overused, if we could get everything that sleep does into a pill, that pill would be a best seller.

But I don’t want to just leave this as an open-ended, “sleep does everything”. I know you want more specifics, so let’s examine the effects of sleep on different systems and processes in the body.

Hormonal Health and What Sleep Does for Vitality

Poor sleep negatively impacts hormonal health in profound ways. Poor sleep negatively affects testosterone, estrogen and growth hormone levels. It also tends to lead to an increase in cortisol levels. This isn’t just about numbers on a blood test, as poorer hormonal health reduces your vitality, reduces your ability to train hard and adapt to that training, reduces your ability to build muscle and get stronger, and makes getting leaner harder.

The hormonal disruption from poor sleep manifests in how you feel throughout your day. Lower testosterone and growth hormone mean reduced capacity for physical work, slower recovery, and diminished sense of well-being. Elevated cortisol means you’re running on stress hormones rather than operating from a place of genuine energy. Your body is compensating for inadequate recovery, not thriving. In general, fertility is also reduced when sleep is poor, which speaks to how fundamentally sleep affects your body’s ability to function optimally.

When sleep is good, these hormones operate in their optimal ranges. Your body produces growth hormone in pulsatile fashion during deep sleep, facilitating tissue repair and growth. Testosterone production occurs primarily during sleep, with levels peaking in the morning after a good night’s rest. This hormonal milieu enables you to train effectively, recover properly, and simply feel like your most energised, capable, and resilient self.

Metabolic Health: What Sleep Does for Blood Sugar and Body Composition

Poor sleep increases your risk for diabetes and obesity through several interconnected mechanisms. A lack of sleep reduces your insulin sensitivity and makes it more likely that you will store calories as fat. When your cells become less responsive to insulin, glucose remains elevated in your bloodstream rather than being efficiently shuttled into cells for energy. This metabolic state mimics pre-diabetes, and chronic sleep deprivation can indeed push you towards developing type 2 diabetes.

Poor sleep also increases the likelihood that stored fat will be visceral fat, which is the worst kind of fat for health. Visceral fat (the fat stored around your organs) is metabolically active and produces inflammatory compounds that increase your risk for cardiovascular disease, metabolic syndrome, and various cancers. This is compounded by a lack of sleep leading to a decrease in leptin (the satiety hormone) and an increase in ghrelin (the hunger hormone), which serves to make you hungrier and thus more likely to overeat.

The metabolic disruption extends beyond just fat storage. Sleep deprivation affects how your body partitions nutrients. Even if you’re eating the same calories, poor sleep pushes your metabolism towards fat storage and away from muscle building. Your body becomes less metabolically flexible, meaning it struggles to efficiently switch between burning carbohydrates and fats for fuel. This inflexibility makes it harder to maintain stable energy throughout the day and increases your reliance on frequent eating to maintain blood sugar levels.

Cardiovascular Health: What Sleep Does for Your Heart

Poor sleep is associated with an increased risk for heart disease through multiple pathways. This is due in part to the effects on metabolic health, but also because poor sleep is associated with increased blood pressure, a worse lipid profile, increased oxidation, increased calcification of the blood vessels and reduced repair of general wear and tear to blood vessels and the heart itself.

During sleep, your blood pressure naturally drops (a phenomenon called “nocturnal dipping”), which gives your cardiovascular system a chance to recover from the demands of the day. When sleep is disrupted or insufficient, this dipping doesn’t occur properly, meaning your heart and blood vessels remain under elevated pressure for extended periods. Over time, this contributes to hypertension and increases strain on your heart.

Sleep also plays a role in maintaining healthy endothelial function (the inner lining of your blood vessels). The endothelium regulates blood flow, blood clotting, and inflammation. Poor sleep impairs endothelial function, making your blood vessels less responsive and more prone to atherosclerosis. The repair processes that occur during sleep help maintain vascular health, and without adequate sleep, this maintenance falls behind, much like a building that’s never properly maintained will eventually deteriorate.

Your lipid profile also suffers with poor sleep. HDL cholesterol (the “good” cholesterol) tends to decrease, whilst LDL cholesterol (the “bad” cholesterol) and triglycerides tend to increase. This shift in lipid balance increases your risk for atherosclerotic cardiovascular disease. The oxidation of LDL cholesterol (a key step in the development of arterial plaques) is also increased when sleep is poor.

Immune Function: What Sleep Does to Protect You

Poor sleep is associated with reduced immune health. With a lack of sleep, your immune system’s ability to fight infections is reduced, and your immune system is somewhat suppressed. It is hard to say exactly how much of an effect sleep has directly on the immune system, but as it acts on other systems that we know affect the immune system (such as the endocrine system), there is reason to believe that poor sleep is reducing immune health.

The connection between sleep and immunity becomes starkly apparent when you consider that sleep deprivation reduces the production of protective cytokines (proteins that help fight infection and inflammation). Certain immune cells, particularly T-cells, are also affected by sleep loss. Studies have shown that people who don’t get adequate sleep are more susceptible to catching colds and other viral infections, and they take longer to recover when they do fall ill.

During sleep, your immune system releases cytokines that help promote sleep but also target infection and inflammation. When you’re sick or injured, you need more of these cytokines, which is partly why you feel sleepier when you’re ill. Your body is trying to get you to rest so it can mount an effective immune response. Chronic sleep deprivation means you’re constantly operating with a compromised immune system, making you more vulnerable to illness and less capable of recovering when you do get sick.

Vaccination responses are also blunted when sleep is poor. Studies have shown that people who are sleep deprived produce fewer antibodies in response to vaccines, meaning the protection offered by vaccination is reduced. This speaks to how fundamental sleep is for immune function, because even when you’re trying to boost your immunity through medical intervention, poor sleep undermines those efforts.

Inflammation: What Sleep Does to Keep Your Body Balanced

Poor sleep is associated with increased inflammation throughout the body. As inflammation is involved in many processes in the body, having an increased level of inflammation due to a lack of sleep is likely not beneficial, especially if you have an inflammatory disease.

Inflammation is a double-edged sword. Acute inflammation is necessary and beneficial; it’s part of your body’s healing response to injury or infection. However, chronic low-grade inflammation, the kind that persists when you’re consistently undersleeping, is damaging. This chronic inflammation is implicated in virtually every chronic disease, from cardiovascular disease to Alzheimer’s to autoimmune conditions.

Sleep deprivation increases inflammatory markers such as C-reactive protein (CRP), interleukin-6 (IL-6), and tumour necrosis factor-alpha (TNF-α). These inflammatory compounds circulate throughout your body, promoting a pro-inflammatory state. This isn’t just about feeling slightly more inflamed; chronic elevation of these markers accelerates ageing and increases your risk for numerous diseases.

For people with existing inflammatory conditions (arthritis, inflammatory bowel disease, psoriasis, and so on), poor sleep can trigger flare-ups and make symptoms worse. The relationship is bidirectional: inflammation can disrupt sleep, and poor sleep increases inflammation, creating a vicious cycle that’s difficult to break. Understanding this connection helps explain why improving sleep often leads to improvements in inflammatory conditions.

Cancer Risk: What Sleep Does for Cellular Health

Poor sleep does increase your risk of cancer, and irregular sleep (as is seen with shift work) is recognised as a carcinogen by the World Health Organisation. A lack of sleep also seems to reduce the effectiveness of cancer treatment.

The mechanisms linking sleep to cancer risk are multiple. Sleep deprivation affects immune surveillance (your body’s ability to identify and destroy abnormal cells before they become cancerous). Natural killer cells, which play a crucial role in identifying and eliminating cancer cells, are significantly reduced with poor sleep. Studies have shown that even a single night of poor sleep can reduce natural killer cell activity by up to 70%.

Melatonin, a hormone produced during sleep, has anti-cancer properties. It acts as an antioxidant and helps regulate cell division. When sleep is disrupted, particularly by light exposure at night (as happens with shift work), melatonin production is suppressed. This disruption has been linked to increased rates of breast cancer, prostate cancer, and colorectal cancer.

The disruption of circadian rhythms (your body’s internal 24-hour clock) is another pathway linking poor sleep to cancer. Your circadian rhythm regulates cell division, DNA repair, and apoptosis (programmed cell death). When this rhythm is chronically disrupted, these processes don’t function properly, increasing the likelihood of cells becoming cancerous. This is why shift workers, who have chronically disrupted circadian rhythms, have elevated cancer risk.

Mental Health: What Sleep Does for Psychological Wellbeing

Most people are probably aware that poor sleep is associated with poorer mental health outcomes. This is pretty much across the board with mental health issues, from depression to anxiety, and across the more severe psychiatric issues like schizophrenia. Poor sleep is associated with worse mental health, and suicide and poor sleep seem to be well correlated. Poor sleep seems to also be linked to an increased risk for (and increased severity of) many neurodegenerative diseases, notably Parkinson’s disease and dementia.

The relationship between sleep and mental health is complex and bidirectional. Poor sleep doesn’t just result from mental health problems; it contributes to them. Sleep deprivation affects neurotransmitter systems, particularly serotonin, dopamine, and norepinephrine, which are all implicated in mood regulation. When these systems are dysregulated due to poor sleep, you’re more vulnerable to developing mood disorders.

Sleep also plays a crucial role in emotional processing and regulation. During REM sleep in particular, your brain processes emotional experiences from the day, helping you maintain emotional equilibrium. When sleep is disrupted, this emotional processing doesn’t occur properly, leaving you more emotionally reactive and less capable of managing stress and difficult emotions.

The connection between sleep and neurodegenerative disease is particularly concerning. During sleep, your brain clears out metabolic waste products, including beta-amyloid (the protein that is thought to accumulate in Alzheimer’s disease). This clearance occurs through the glymphatic system, which is most active during sleep. Chronic sleep deprivation means this waste clearance doesn’t occur adequately, potentially contributing to the accumulation of toxic proteins in the brain over time.

For people already struggling with mental health issues, improving sleep can be transformative. Whilst it’s not a cure-all, better sleep often leads to significant improvements in mood, anxiety, and overall mental well-being. This isn’t surprising when you consider that sleep affects virtually every aspect of brain function.

Cognitive Performance: What Sleep Does for Your Mind

Poor sleep decreases your general mental performance, and in an increasingly knowledge-based economy, this only serves to reduce your job performance and thus income prospects. Lower income is a risk factor for poorer health outcomes, creating yet another feedback loop where poor sleep undermines multiple aspects of your life.

The cognitive effects of sleep deprivation are wide-ranging and significant. Attention and concentration suffer, meaning you find it harder to focus on tasks, and you’re more easily distracted. Your working memory is impaired, making it difficult to hold information in mind whilst you’re working with it. This affects everything from following conversations to solving problems to learning new information.

Decision-making is also compromised when you’re sleep-deprived. You become more impulsive, less able to think through consequences, and more likely to take unnecessary risks. Your ability to evaluate options and make sound judgements deteriorates. This affects not just big life decisions, but the countless small decisions you make throughout each day: what to eat, how to respond to a colleague, whether to push through with a task or take a break, etc.

Reaction time slows significantly with sleep deprivation, to the point where being awake for 24 hours produces impairment equivalent to being legally drunk. This is why sleep-deprived driving is so dangerous; your ability to react to changing road conditions is severely compromised. But it’s not just reaction time for physical movements that suffers; your cognitive reaction time (how quickly you can process information and respond appropriately) is also impaired.

Creativity and problem-solving abilities decline with poor sleep. Sleep, particularly REM sleep, facilitates creative insight and the ability to see novel connections between ideas. When you’re well-rested, you’re better able to think flexibly, generate creative solutions, and see problems from multiple perspectives. Sleep deprivation makes your thinking more rigid and less innovative.

Learning capacity is also significantly affected by what sleep does. Sleep is crucial both for encoding new information (learning it in the first place) and consolidating that information into long-term memory. When you’re sleep deprived, both processes suffer. You struggle to learn new things, and the things you do manage to learn are less likely to stick. This is why cramming all night before an exam is such a poor strategy. You might get some information into short-term memory, but without sleep to consolidate it, much of it will be lost.

Injury Risk: What Sleep Does for Safety

Poorer sleep is associated with an increased risk of injury, both in everyday life and when engaging in sports and other activities. This could potentially severely reduce your ability to interact with the world (you could become disabled!), or it could even drastically reduce your life span if injuries require multiple surgeries, increasing your risk for infection and other complications.

The mechanisms linking sleep to injury risk are multiple. Reduced cognitive function means slower reaction times and poorer decision-making, making accidents more likely. Impaired coordination and balance make you more prone to trips, falls, and other physical mishaps. Your ability to assess risk is compromised, leading you to take chances you wouldn’t normally take or fail to recognise dangerous situations.

In sports and physical activities, sleep deprivation significantly increases injury risk. Your proprioception (your sense of where your body is in space) is impaired, making you more likely to move incorrectly or fail to land properly. Your muscles don’t coordinate as well, increasing the likelihood of strains and sprains. Your pain tolerance is reduced, but paradoxically, your awareness of your body’s signals is also diminished, meaning you might push through warning signs that would normally make you stop.

This is particularly important in the context of building a life you want to live. An injury can derail your training, your recreational activities, your ability to work, and your capacity to care for others. Protecting yourself from injury isn’t about being overly cautious, it’s about maintaining the physical capacity to engage fully with your life. Sleep is a crucial part of that protection.

Longevity and Healthspan: What Sleep Does for Your Future Self

All of these effects of what sleep does compound over time to affect both your lifespan and healthspan (the number of years you live in good health, capable of doing what matters to you). Poor sleep is associated with increased all-cause mortality, meaning people who consistently sleep poorly die younger from all causes combined.

But perhaps more importantly, poor sleep affects your healthspan. You might live a long time, but if those later years are characterised by chronic disease, disability, and dependence on others, that’s not the kind of longevity most people aspire to. Sleep is fundamental to maintaining the physical and cognitive capacity to live independently and engage meaningfully with life as you age.

The effects of sleep on longevity and healthspan aren’t just about avoiding disease, although that’s certainly part of it. They’re about maintaining resilience and the ability to bounce back from setbacks, adapt to change, and continue growing and learning throughout your life. Sleep is foundational to that resilience.

Now, all of that can be quite sobering, and I don’t mean for it to be excessively alarming. It is just important to understand that poor sleep does have many ramifications for our health. As you can see, poor sleep is associated with many negative health outcomes, and we didn’t even cover them all. But I just want to remind you that the magnitude of effect is likely different for someone who occasionally gets a little bit less sleep than they would like, compared to someone who only sleeps four hours a night regularly. Don’t be afraid that just because you got seven and a half hours of sleep last night rather than eight, you are now doomed to ill health. For sure, work to improve your sleep, but don’t think that the body is fragile and can’t handle the occasional bit of sleep deprivation.

Sleep does also have effects on the other pillars of health (diet, exercise and stress), and these pillars also have effects on sleep. And because this is the stuff that we have more control over, it really does help to understand what is going on with these other areas.

What Does Sleep Do for Athletic Performance and Training?

Sleep doesn’t just affect your health, it also profoundly affects your ability to train, perform, and adapt to exercise. In general, poor sleep is associated with worse training outcomes and worse training performance. Most people understand this fairly intuitively, as sleep is important for just generally feeling well-rested and energised. If your sleep is poor and thus you aren’t well rested and energised, well, your performance during exercise is likely to be reduced. Similarly, sleep is important for repair and recovery, and thus, even if you do train hard and your performance is decent, you are getting less repair and recovery from that session when sleep is poor. It may take you longer to recover from that training session, and this may compound over time into poorer results. However, there is more to this story than just this more intuitive understanding.

Strength, Power, and What Sleep Does for Physical Output

Poor sleep results in an altered metabolism (as discussed previously), and this generally results in a pattern of metabolism that is not favourable to exercise performance. Effectively, with a lack of sleep, fat storage is being prioritised, and carbohydrates aren’t readily getting into your cells, and thus your preparedness for exercise is reduced. As muscle glycogen is unlikely to be depleted in everyday life, this lower exercise performance may not impact shorter, higher intensity work as much, but longer duration work does seem to be quite negatively affected.

Naturally enough, if poor sleep is a chronic thing, you will be more likely to notice decreases in performance. Your maximum strength (i.e. the amount you can lift for a single repetition) may not be dramatically affected by a single night of poor sleep, but your ability to perform multiple sets, maintain force output across a training session, and recover between workouts certainly is.

It is important to note that exercise does somewhat help improve the negative effects seen from poor sleep, as it improves your ability to get carbohydrates into the cells, and it can also improve lipid metabolism. So we do generally recommend still exercising when sleep is poor (unless it further interferes with your ability to get good sleep that night), however, we also generally recommend just taking it a bit easier whilst training and not doing an excessive amount of volume and also leaving more reps in the tank (not lifting as close to failure).

Some people do actually notice a performance increase when they are somewhat sleep deprived, and this is usually only really seen in the shorter duration events (such as lifting weights for low reps or sometimes even sprinting). This is likely due to poor sleep causing an increase in stress hormones, which are serving to get you a bit more hyped up for the short duration events. Generally, even in these individuals, you will see that their total exercise volume is reduced with poor sleep in the mix. They may hit a new five rep max personal record, but they find that they can’t do their normal three to four sets of each exercise on their programme, or they can’t finish the workout in full.

Endurance, Skill Acquisition, and What Sleep Does for Athletic Development

Poor sleep does also lead to greater ratings of perceived exertion, so even doing the same work as last time feels harder. This likely decreases your desire to train, especially if you are chronically getting poor sleep. It is hard to be motivated to train when it feels like you are not only not progressing, but you are actually regressing and things are getting harder, not easier.

Pain tolerance is also reduced with poor sleep, so your ability to push training into productive territory is likely reduced. Again, if the poor sleep is a chronic thing, you will find it difficult to progress if training feels harder than it should, getting closer to failure is just more painful (subjectively, you feel it is more painful due to lower pain tolerance), and you experience more pain after the workout too. Delayed onset muscle soreness feels more painful than usual.

Cognition and your ability to focus and concentrate are negatively affected when your sleep is poor. Coordination and motor function are also negatively affected when sleep is poor. This makes training effectively much harder, but it also likely increases your risk of injury. This is especially true of more skill-requiring movements, and even more so when your training is in the less controlled environment of sport. It is also harder to learn and perform complex movements when you are sleep deprived, and this is obviously an issue for those who do sports as their exercise (although it can be somewhat overcome in the gym by simply choosing less complex movements).

The learning component is particularly important for what sleep does for athletic development. When you’re learning a new skill (whether that’s a complex Olympic lift, a football technique, or a dance routine), sleep is when that skill gets consolidated. During sleep, your brain rehearses and refines motor patterns, strengthening the neural pathways that encode those skills. Without adequate sleep, this consolidation doesn’t occur properly, meaning you learn new skills more slowly and the skills you do learn are less refined.

Recovery, Adaptation, and What Sleep Does for Training Results

With poor sleep, you may not even make it to training, because poor sleep is associated with reduced motivation. This obviously isn’t an ideal situation when consistency is the biggest factor in determining the results you can get from training.

If you do make it to training, you then get hit with poorer recovery from that training. Less muscle growth, less strength acquisition, reduced improvements in cardiovascular function and slower recovery from training overall. Not really ideal at all. Now, of course, there is clearly a difference between a bit of poor sleep one night and chronic sleep deprivation, so the magnitude of this point may not actually be that big of a deal, although it does serve to highlight the fact that sleep is an important part of the adaptation to exercise and the results you get from it.

What sleep does for training adaptation is perhaps one of its most important functions for anyone who exercises. Training is stress: you’re deliberately damaging muscle fibres, depleting energy stores, and challenging your cardiovascular and nervous systems. The actual adaptations (building bigger, stronger muscles, improving cardiovascular capacity, enhancing neural efficiency) don’t occur during training, they occur during recovery, and sleep is when the bulk of that recovery happens.

During sleep, muscle protein synthesis rates are elevated, helping to repair damaged muscle fibres and build new muscle tissue. Growth hormone, which is released primarily during deep sleep, facilitates this process. Testosterone, which is also produced during sleep, plays a crucial role in muscle building and strength development. When sleep is poor, these hormonal signals are disrupted, meaning the raw materials and signalling molecules needed for adaptation aren’t present in optimal amounts.

Similar to sleep’s effects on diet and health, getting a more optimal amount of sleep leads to a more favourable return from exercise. However, unlike sleep’s effects on the diet, you may actually get a bit more return on investment from exercise if you do sleep a bit more. Nothing extraordinary, but if you are really trying to maximise your returns from exercise, then getting a bit more sleep than is generally recommended may be of benefit.

Now, I realise that discussing the negative effects of poor sleep on exercise (and in general) can be quite off-putting. However, I don’t want you to think that you are doomed to get terrible results from training if your sleep isn’t perfect, and I certainly don’t want to discourage anyone from engaging in exercise just because they can’t get their sleep perfect. In fact, oftentimes, getting some sort of exercise done does actually improve the negative consequences of poor sleep and exercise is consistently shown to improve sleep. This is because exercise and sleep have a bidirectional relationship, and your exercise patterns do influence your sleep too.

Exercise and Sleep: The Bidirectional Relationship

Engaging in exercise is actually one of the best things you can do for your sleep, as exercise is consistently associated with improvements in sleep parameters. It tends to improve various sleep parameters, such as reducing the time it takes to get to sleep and improving overall sleep quality. Lifting weights, doing anaerobic training, doing aerobic training, doing sports and pretty much everything in between all seem to have positive effects on sleep parameters. Whilst you can get a bit more granular with exercise type and the exact improvements to sleep you want to elicit, in reality, this isn’t necessary, and just exercising in any shape or form does tend to improve sleep (once it is done for sufficient intensity and duration). This is both an acute thing (sleep is improved the same day you exercise) and a more chronic thing (the improvements occur over time when you are consistently exercising).

However, it does bear noting that exercising close to bedtime does seem to make it harder to get to sleep for a lot of people. This is both because exercise is generally something that is quite arousing (it gets you more “hyped up”) and you are quite switched on after training (this is especially the case if you do stuff that is more neurologically demanding, such as heavy lifting or sprinting). But it is also because your body temperature is likely increased after training (and as we will discuss in future articles on sleep regulation, this is one of the things that plays a role in getting to and staying asleep). So in general, it isn’t advisable to exercise in the couple of hours before bedtime. If that is the only time you can exercise, then that isn’t the end of the world, but it would make sense to try and not get overly hyped up for those sessions and also focus on cooling the body down after exercise.

In general, training in the morning or afternoon is likely better for allowing you to really reap all the benefits of the sleep improvements that exercise offers. However, that is likely impractical for the vast majority of you, as that is usually when you have other stuff going on. In the morning you may be rushing to get the children to school and get to work yourself, and then during the day you are at work, and that really only leaves the evening to exercise for a lot of people. This isn’t a huge issue, but if you can leave some time after training to wind down, that will likely lead to better sleep quality than if you were to exercise very close to bedtime. If you are training in the morning and cutting your sleep short to do so, well, this is likely less than ideal too. But in the real world, sometimes you have to choose the less than optimal course of action.

What also bears noting is that excessive exercise can also result in poorer sleep. This generally isn’t an issue that most people will have to deal with, but some athletes training multiple hours per week can actually reach a state of overtraining, where their sleep quality will actually decrease, and some even develop symptoms of insomnia. Now, this usually isn’t something to worry about, as most people simply don’t exercise enough to ever even get close to this, but it does bear noting that there is a bit of a Goldilocks zone with exercise for sleep too.

What Does Sleep Do for Body Composition?

Sleep and diet are intrinsically linked, and it seems to be a bidirectional link. Sleep impacts the diet, and the diet impacts sleep. We discussed metabolism in previous sections, so I won’t bore you with a full discussion of the basics again. To make a long story short, sleep does actually influence our metabolism pretty profoundly. It impacts both our calorie expenditure (calories out) and our calorie intake (calories in).

Energy Expenditure and What Sleep Does to Your Metabolism

When you sleep less, you end up burning fewer calories during the day at a baseline. Poor sleep results in disturbed hormones, notably thyroid hormones. Lower thyroid output leads to a lower metabolic rate. However, the bigger effect is likely due to the simple fact that being tired likely reduces your energy expenditure by reducing your output during training and your daily life. Reduced exercise activity thermogenesis (EAT) and non-exercise activity thermogenesis (NEAT) both contribute to lower overall energy expenditure.

Now, the overall impact may not ultimately add up to much, but it is important to realise that your metabolism and calorie expenditure is likely a little bit lower if you are sleep deprived. The effect could be quite large though, especially if sleep deprivation frequently leads to training being skipped due to tiredness or daily NEAT (as measured by steps, for example) being very low. If you’re consistently moving less because you’re tired, the cumulative effect on energy expenditure can be substantial.

Appetite Regulation and What Sleep Does to Your Hunger

A bigger effect of poor sleep seems to be its effect on influencing your calorie intake. Poor sleep is associated with an increased energy intake (calories in). Most of you have likely experienced this, where you get a poor night of sleep and you just can’t seem to feel full, and you are just constantly hungry all day as a result. This is quite common, and obviously, feeling excessively hungry drastically decreases the likelihood that you will be able to stick to your diet and eat a calorie-appropriate diet long term.

The mechanisms behind this increased hunger are hormonal. Sleep deprivation decreases leptin (the hormone that signals satiety) and increases ghrelin (the hormone that signals hunger). But it’s not just these two hormones. Poor sleep also affects other appetite-regulating hormones, including peptide YY and cholecystokinin. The net result is that your body is telling you you’re hungrier than you actually are from an energy balance perspective.

Poor sleep is also associated with poorer mood regulation, and thus your dietary adherence may also be reduced by virtue of simply feeling worse and turning to food for comfort. When you’re tired, you have less willpower to resist cravings and less capacity to make thoughtful food choices. The prefrontal cortex (the part of your brain responsible for executive function and impulse control) is particularly sensitive to sleep deprivation. This means that even if you know you shouldn’t eat that extra biscuit, your ability to override the impulse is compromised.

Food reward is also altered when you’re sleep deprived. Foods high in sugar and fat become more rewarding, more appealing, and more difficult to resist. Your brain is seeking quick energy to compensate for your depleted state, and highly palatable, energy-dense foods light up reward centres more intensely when you’re tired. This isn’t a character flaw or lack of discipline, it’s your brain’s survival mechanism trying to get you the energy it thinks you need.

Nutrient Partitioning and What Sleep Does to Body Composition

But even if you were able to stick to your calories despite your poor sleep, you will likely be getting less bang for your buck from the diet. You see, a lot of the hormones that are disrupted when sleep is in a bad place are the hormones that play a role in how the food you eat is used. Carbohydrate metabolism is negatively affected when sleep is bad, pushing you towards a more insulin-resistant state. This isn’t great from a health perspective or a muscle-gain/fat-loss perspective either.

Fat metabolism is also disrupted when sleep is poor, which leads to faster storage of fat for later use (a calorie surplus is still required to add to your fat stores over time). But with reduced calorie expenditure and potentially increased food intake due to poor sleep, fat gain is more likely. The fat that is stored in this situation also tends to be visceral fat (the fat stored around your organs), which is the worst type of fat for health.

Finally, protein metabolism is also affected by poor sleep. Protein synthesis is reduced (so fewer protein structures, such as muscle, are built), and protein degradation is increased (more protein is broken down). This is particularly problematic if you’re trying to build muscle or even just maintain muscle whilst losing fat. Your body is simultaneously less capable of building muscle and more prone to breaking it down.

Ultimately, poor sleep leads to reduced muscle anabolism (muscle building), increased fat anabolism (fat storage), increased muscle catabolism (muscle breakdown) and decreased fat catabolism (fat breakdown). Whilst this is a bit of a simplification, and there is obviously a difference between missing an hour of sleep and chronically undersleeping, the general effect is an unfavourable one for those of us who care about our health, performance and body composition.

Getting good quality sleep brings all of these parameters back into a more advantageous position. However, getting more sleep than is needed (unfortunately) doesn’t confer massive improvements in these various parameters. There is a Goldilocks zone with sleep, and getting more sleep than you need isn’t likely to improve your health beyond a certain point, and may just be detrimental to your overall well-being. Sleeping for ten-plus hours each day does actually eat into your ability to do other things with your life.

Diet and Sleep: What You Eat Affects How You Sleep

Now, we have covered what happens to the diet when sleep is poor, but I did note that this is a bidirectional pathway. What you eat also affects your sleep. We will cover this a bit more practically in future articles on sleep hygiene, but it makes sense to cover it briefly here too.

Protein intake seems to play a role in good quality sleep. Higher protein diets seem to be correlated with improved sleep quality. So it makes sense to ensure that protein intake is sufficient.

Carbohydrates are anecdotally associated with better sleep quality and also with helping people get to sleep faster. There are many potential mechanisms for carbs helping you to fall asleep faster, but it is quite an individualised thing and doesn’t seem to be consistent across people. However, if you notice that eating more carbs improves your sleep, then that is obviously a tool in your toolbox to use to improve your sleep.

However, what does seem to consistently lead to poorer sleep is having insufficient fuel to last through the night. Some individuals who are more insulin resistant may find that unless they eat carbs before bed, they will wake up due to dropping blood sugar levels. Similarly, individuals who are eating in a calorie deficit may also experience poorer sleep due to low blood sugar levels, general hunger being higher and potentially also higher stress levels (which are serving to mobilise stored fuel). Obviously, if you have less fat stores and you are in a greater deficit, the sleep disturbances may be more pronounced.

Some people say they sleep better when they eat before bed, however, the research does seem to suggest that eating in the couple of hours before bed isn’t the best practice for improving sleep (or your health). It isn’t a massive issue if you do eat before bed, especially if the vast majority of your lifestyle and sleep practices are dialled in, but it is quite common for sleep quality to be lowered when eating before bed. You may get to sleep more quickly with a belly full of food, but the recovery and repair processes that are supposed to happen when you are asleep can’t occur until the food in your stomach is dealt with. So, leaving a bit of a gap between the last meal and sleep makes sense.

Being poorly hydrated also can lead to poorer sleep, which is a tricky one to deal with, as some people find that drinking water in the lead-up to sleep increases the likelihood of them having to get up to urinate during the night. So you have to ensure hydration is in a good place throughout the day, if you want to ensure hydration is sufficient across the night. This is especially important if you live in a warmer climate (or you create a warmer microclimate with multiple clothing layers, blankets and heating) and this causes you to sweat through the night. Hydrating through the day and then perhaps tapering off your water intake closer to bed likely leads to improved sleep quality.

Coffee intake is also associated with poorer sleep, especially if that coffee intake is later in the day. Caffeine is a stimulant, and thus it is not conducive to high-quality sleep or to helping you get to sleep quickly. The unfortunate thing is that caffeine does actually stay in your system for quite a long time, especially if you are a slow caffeine metaboliser. Even if you can get to sleep, if you have an excessive amount of caffeine in your system, your sleep quality will likely be reduced. Generally, consuming less than 400mg of caffeine in a day, and having a cut-off for coffee nine to ten hours before bed is best practice, but even this is associated with some reductions in sleep quality.

Alcohol is another drink that negatively affects sleep. Some people will argue that some alcohol is good for sleep as it helps them to get to sleep a bit faster, but even if this were the case, their sleep quality is still reduced. Even with one drink, sleep quality is reduced, so it is best to avoid alcohol if you care about your sleep.

Finally, eating foods that just don’t sit well with your digestive system is likely going to negatively impact your sleep. You are unlikely to get high-quality sleep if your stomach is doing somersaults all night. So choosing foods that sit well with your digestion and don’t leave you feeling gassy, bloated, or generally unwell is probably a good idea.

What Does Sleep Do for Mental Performance?

We touched on this in the health section, but mental performance deserves its own deeper examination because what sleep does for cognitive function affects every aspect of how you engage with your life.

Memory, Learning, and What Sleep Does for Skill Development

Memory consolidation is one of the most well-established functions of what sleep does. During sleep, particularly during specific stages of sleep, your brain processes and consolidates memories from the day. This isn’t a single process and different types of memories are consolidated during different stages of sleep.

Declarative memory (your memory for facts and events) is consolidated primarily during slow-wave sleep (deep sleep). When you learn new information during the day, it initially exists in a fragile, temporary state in the hippocampus. During sleep, this information is replayed and transferred to more permanent storage in the cortex. Without adequate sleep, this consolidation doesn’t occur properly, and the information is more likely to be forgotten.

Procedural memory (your memory for skills and procedures) is consolidated primarily during REM sleep. This is why sleep is so crucial for learning motor skills, musical instruments, sports techniques, and any other skills that involve “knowing how” rather than “knowing that”. During REM sleep, your brain rehearses these motor patterns, strengthening the neural pathways that encode them.

Emotional memory is also processed during sleep, particularly during REM sleep. Your brain doesn’t just consolidate emotional memories, it also processes and regulates them, helping you maintain emotional equilibrium. This is why “sleeping on it” often helps with emotional difficulties. The emotional charge of an experience is often dampened during sleep, allowing you to remember the event without being overwhelmed by the emotion.

Creativity, Problem-Solving, and What Sleep Does for Innovation

What sleep does for creativity and problem-solving is remarkable. There’s a reason why people often wake up with solutions to problems they couldn’t solve the previous day. During sleep, particularly during REM sleep, your brain makes novel connections between disparate pieces of information. Neural activity during REM sleep shows that your brain is highly active and is making associations that wouldn’t occur during waking consciousness.

This is why sleep is so crucial for creative work, whether that’s artistic creativity, scientific innovation, or simply finding creative solutions to everyday problems. When you’re stuck on a problem, getting a good night’s sleep often provides the insight you need. This isn’t magic, it’s just your brain doing what it does during sleep: making connections, integrating information, and seeing patterns.

The way sleep facilitates creativity is through a process called memory integration. Information from different domains that seemed unrelated whilst you were awake gets connected during sleep. Your brain essentially plays with ideas, trying different combinations and associations without the constraints of waking logic. This is why dreams can be so bizarre, your brain is exploring possibilities without the usual filters.

Executive Function and What Sleep Does for Self-Regulation

Executive function (your ability to plan, organise, manage time, inhibit impulses, and regulate behaviour) is profoundly affected by what sleep does. The prefrontal cortex, which is responsible for executive function, is particularly sensitive to sleep deprivation. Even modest sleep loss impairs executive function significantly.

This matters enormously for living well. Executive function is what allows you to override immediate impulses in favour of long-term goals. It’s what enables you to stick to your diet when tempted by sweets, to go to the gym when you’d rather stay on the sofa, to work on important projects rather than scroll social media, to respond thoughtfully rather than react emotionally.

When executive function is impaired by poor sleep, you become more impulsive, less capable of self-regulation, and less able to act in accordance with your values. You’re more likely to make decisions you’ll regret, to give in to temptations, to say things you don’t mean. Your capacity for self-control is finite, and it’s significantly reduced when you’re sleep deprived.

This is one of the ways poor sleep undermines your agency. You might think you’re making free choices, but when your executive function is compromised by sleep deprivation, you’re not operating with full capacity. You’re more at the mercy of impulses, habits, and immediate circumstances. Getting adequate sleep restores your capacity for self-direction and intentional action.

Emotional Regulation and What Sleep Does for Equanimity

What sleep does for emotional regulation is closely tied to executive function but deserves special attention. Sleep deprivation makes you more emotionally reactive and less capable of managing your emotional responses. The amygdala (the part of your brain involved in emotional responses, particularly fear and anger) becomes hyperactive when you’re sleep deprived, whilst the prefrontal cortex’s ability to regulate the amygdala is impaired.

This means that when you’re tired, things bother you more than they normally would. You’re more irritable, more prone to anger, more likely to feel anxious or overwhelmed. Small stressors feel like major problems. Your emotional responses are disproportionate to the situations that trigger them.

This affects your relationships, your work, and your ability to navigate daily life. When you’re sleep deprived, you’re more likely to snap at your partner, to feel frustrated with your children, to overreact to feedback from your boss, and to catastrophise about minor setbacks. You know intellectually that you’re overreacting, but you can’t seem to help it. That’s your sleep-deprived amygdala overwhelming your impaired prefrontal cortex.

Getting adequate sleep restores emotional regulation. You’re better able to maintain equanimity in the face of difficulties, to respond proportionately to challenges, to give others the benefit of the doubt. You’re simply more pleasant to be around and more capable of maintaining the relationships that make life worth living.

What Does Sleep Do for Stress?

Sleep and stress are intimately connected, and understanding what sleep does for stress (and what stress does to sleep) is crucial for breaking negative cycles and building positive ones.

Stress Capacity and What Sleep Does for Resilience

We often use the analogy of a bucket of water to describe your stress. The bucket gets filled with the various stressors in your life, and the more stress in the bucket, the harder it is to carry and balance. When sleep is poor, you generally have a reduced capacity to handle stress. This is akin to saying poor sleep reduces the capacity of your bucket, and thus it is easier for a smaller amount of stress to fill the bucket.

This should be intuitive enough, as most people have likely felt that they just couldn’t handle as much stress when their sleep wasn’t in a good place. Poor sleep is a stressor itself, and thus it takes up space in the bucket. Your baseline stress levels are higher when sleep is poor, and unfortunately, that just leaves less room for the other stressors in your life.

What sleep does for stress tolerance is multifaceted. Physiologically, sleep helps regulate your hypothalamic-pituitary-adrenal (HPA) axis (your body’s main stress response system and a key part of your overall hormonal system). When this system is well-regulated, you respond appropriately to stressors and then return to baseline once the stressor is gone. When sleep is poor, the HPA axis becomes dysregulated, leading to elevated cortisol, increased inflammatory responses, and a persistent state of physiological stress.

Poor sleep also reduces your recovery from exposure to stressors, and thus, the bucket doesn’t get emptied out as much overnight. Sleep is when your body recovers from the stressors of the day, both physical and psychological. Without adequate sleep, you start each day with residual stress from the previous day, and it accumulates over time.

Stress Management and What Sleep Does for Coping

Further to this, poor sleep does actually make it harder to engage with the various stress management practices that serve to reduce stress. It also reduces your cognition and executive function, so you potentially make poorer decisions around stress management practices, and most people fail to even identify that a lack of sleep is causing their stress levels to be higher than normal.

When you’re tired, life feels harder, exercise feels like too much effort, cooking healthy meals feels overwhelming, and even simple stress management practices like going for a walk or calling a friend feel like insurmountable tasks. Your capacity to do the things that would help you feel better is reduced precisely when you need them most.

Most people also reach for caffeine when their sleep is poorer, and caffeine (and other stimulants) are less than ideal when stress is higher. Excessive caffeine consumption also reduces sleep, which only serves to increase stress levels if this pattern continues. Frequently, people also reach for alcohol in this situation too, but unfortunately, alcohol is a stressor itself, and it reduces sleep quality. It is very common for people to find themselves in the negative cycle of getting poor sleep, drinking lots of coffee to combat the lack of sleep, then drinking a few glasses of wine or bottles of beer each night to reduce the feelings of stress, and then finding they wake up the next morning in an even worse position because their sleep quality was terrible and the pattern continues.

Breaking this cycle requires recognising it first. What sleep does is provide the foundation that makes it possible to engage in effective stress management. When sleep is adequate, you have the energy and mental clarity to make good decisions, the discipline to stick to helpful practices, and the resilience to handle life’s inevitable difficulties.

Stress and Sleep: The Bidirectional Relationship

As noted previously, stress does also affect your sleep. Most of you who have gone through periods of higher stress have likely experienced this in some way. Being highly stressed generally leads to one, some or all of the following issues with sleep: an inability to get to sleep easily, frequent wakings throughout the night, waking up early in the morning and being unable to get back to sleep, and struggling to get into deeper sleep.

In general, stress has a negative impact on sleep. This is a bit more complex, as stress is a complex thing, and we will cover this more in future articles. Some level of stress is actually good for sleep, and stressful things like training do lead to improved sleep. However, various types of stress and the timing of that stress do lead to poorer sleep.

What is interesting to note at this stage is that what most people do to improve sleep is actually just stress management. Many of the sleep hygiene practices we will outline in future articles do actually serve to help with stress management. This is because stress and sleep are very intertwined, but also because a lot of people think they have a sleep issue when in reality they have a stress issue.

This isn’t always the case, and working on improving sleep does also help with improving stress. However, when you ask people what they are struggling with, with regard to their sleep, very often they discuss things that are actually stress-related issues. For example, many people will say they struggle to get to sleep because their mind is racing and they are going through a million and one scenarios in their head. While basic sleep hygiene practices can help with this, stress management practices do seem to be more helpful in this case.

Of course, the combination of good sleep hygiene and good stress management (and good diet and exercise) practices is even better. As a result, you will see that some of the basic sleep hygiene practices we’ll discuss in future articles do actually serve to improve stress management, and thus hopefully improve sleep. But in some cases, more comprehensive stress management practices will need to be engaged with to improve sleep.

The Interconnected Nature of What Sleep Does

So, we have covered a lot in this article. I hope this has given you a better picture of what sleep actually does, at least regarding the areas we care most about. When you look online, most of the content you find just stops at “sleep is important for rest and recovery”, which is helpful and certainly true, but it doesn’t really help us understand what sleep does. I hope that by covering the effects of both good sleep and poor sleep, you now have a better understanding of what sleep does and why it matters so profoundly.

However, even this really only scratches the surface of things. Sleep affects virtually every system in your body, every aspect of your mental function, and every domain of your life. But luckily for us, we don’t need to become sleep scientists to get good-quality sleep. What we need is to understand enough about what sleep does to take it seriously and prioritise it appropriately.

The Feedback Loops: When Things Go Well and When They Don’t

What I hope you have grasped from this discussion is a better appreciation of how intertwined your sleep, diet, exercise and stress all are. They really do feed back into each other quite a bit, and this is both a blessing and a curse.

It is a blessing in so far as, once you start getting some momentum going with good habits and you start improving your sleep, diet, exercise and stress management practices, you generally start to see everything else improve, which then makes it easier to continue improving all of the pillars. Better sleep makes it easier to exercise, which improves sleep further. Better sleep makes it easier to stick to your diet, which improves your body composition and metabolic health, which improves sleep. Better sleep increases your stress resilience, which makes it easier to engage in stress management practices, which improves sleep. This positive feedback loop is what makes sustained behaviour change possible; you’re not fighting uphill forever, because each improvement makes the next one easier.

However, the curse is that if you neglect one of the pillars, you get worse results from the work you are doing on the other things. Poor sleep undermines your training, makes diet adherence harder, and reduces your stress tolerance. High stress disrupts your sleep, makes healthy eating more difficult, and impairs your training recovery. The negative feedback loops can be just as powerful as the positive ones.

So you kind of have to work on everything at once, and this can feel quite overwhelming. However, the really important thing to note is that you don’t have to be perfect with everything from the start, and once you start getting some momentum going with the habits you are engaging in to improve one of the pillars, the easier it does get. Focusing on those small wins and looking for that one per cent improvement does really add up with this stuff, and over time, it becomes easier and easier to stay in the groove with things.

This is where most people fail: they don’t stick with things long enough to build that momentum on their habits, and thus they never get to see that it does get easier once you have some momentum pushing you forward with all of this stuff. The interconnected nature of what sleep does means that improving your sleep creates ripples throughout your entire life, making improvements in other areas more accessible and sustainable.

Individual Variability in What Sleep Does

It’s worth noting that there is significant individual variability in how people respond to sleep and sleep deprivation. Some people seem remarkably resilient to poor sleep, whilst others struggle dramatically after even modest sleep loss. This variation is partly genetic, partly due to age, partly due to overall health status, and partly due to training and conditioning.

Genetic factors influence how much sleep you need, how deeply you sleep, and how well you tolerate sleep deprivation. There are genetic variants that affect circadian rhythms, sleep pressure, and sleep architecture. Some people are genuine “short sleepers” who function well on less sleep than average, though these individuals are much rarer than people think. Most people who believe they function well on limited sleep are actually chronically sleep deprived and don’t realise how impaired they are because they’ve adapted to feeling suboptimal.

Age also plays a significant role in what sleep does and how much you need. Children and adolescents need substantially more sleep than adults, and their sleep serves crucial developmental functions. Older adults often sleep less than younger adults, though whether this represents decreased need or decreased ability to sleep is debated. What’s clear is that sleep architecture changes with age, with less deep sleep and more fragmented sleep being common in older adults.

Sex differences exist in sleep as well. Women often report more sleep problems than men, though they actually spend more time in deep sleep when measured objectively. Hormonal fluctuations across the menstrual cycle, during pregnancy, and during menopause all affect sleep. Men, particularly older men, are more prone to sleep apnoea, which dramatically impairs sleep quality even if total sleep time seems adequate.

Training status also affects how you respond to sleep and sleep deprivation. Well-trained athletes often have different sleep patterns than sedentary individuals, and they may be more resilient to some effects of sleep deprivation, whilst being more vulnerable to others. The increased training load creates additional recovery demands, potentially increasing sleep needs.

Despite this individual variability, the fundamental functions of what sleep does remain consistent across individuals. Everyone needs adequate sleep for optimal health, performance, and mental function, even if the specific amount and timing vary somewhat between people.

Acute Versus Chronic Sleep Deprivation

We can’t talk about what sleep does without discussing what happens when you don’t sleep. However, it’s important to distinguish between acute sleep deprivation (missing a night of sleep or getting significantly less sleep than usual for a night or two) and chronic sleep restriction (consistently getting insufficient sleep over weeks, months, or years). What sleep does, and what happens when you don’t get enough, differs significantly between these scenarios.

Acute sleep deprivation has dramatic, immediately noticeable effects. After missing a night of sleep, your cognitive function is severely impaired, your physical performance suffers, and you feel obviously terrible. However, these effects are largely reversible with recovery sleep. A couple of good nights of sleep and you’re mostly back to baseline.

Chronic sleep restriction is more insidious. The effects accumulate gradually, and you often don’t notice how impaired you’ve become because you’ve adapted to feeling suboptimal. You might think you’re functioning fine on six hours of sleep per night, but objective measures show significant deficits in cognitive performance, physical performance, metabolic health, and immune function. What’s particularly concerning is that subjective sleepiness stabilises after a few days of restricted sleep, even as objective performance continues to deteriorate. You stop feeling tired, but you’re still impaired.

The health consequences of chronic sleep restriction are also different from acute sleep deprivation. It’s the chronic pattern that increases your risk for cardiovascular disease, diabetes, obesity, cancer, and neurodegenerative disease. Occasional nights of poor sleep aren’t pleasant, but they don’t substantially increase your long-term health risks.

Recovery from chronic sleep restriction is also different from recovery from acute sleep deprivation. You can’t simply “catch up” on sleep by sleeping longer on weekends. Whilst recovery sleep does help, it takes consistent adequate sleep over weeks to fully reverse the effects of chronic sleep restriction. Sleep debt is real, but paying it back requires sustained good sleep, not just a few long lie-ins.

Understanding this distinction is important for calibrating your response to poor sleep. If you have a bad night, don’t catastrophise, just get back to your normal sleep routine as soon as possible. But if you’ve been chronically undersleeping, recognise that it will take time to fully recover, and commit to sustained improvement rather than expecting immediate results.

What Sleep Does for Living Well

Throughout this exploration of what sleep does, we’ve examined the effects on physical health, athletic performance, body composition, mental performance, and stress resilience. But it’s worth stepping back to consider what all of this means for living a good life.

Sleep is not an end in itself. Getting adequate sleep doesn’t mean you’ve achieved something meaningful, it simply means you’ve maintained the foundation that makes meaningful achievement possible. Sleep is about capacity. It’s about having the physical energy to engage fully with your day, the cognitive clarity to think clearly and make good decisions, the emotional regulation to maintain important relationships, and the resilience to handle life’s inevitable difficulties.

When sleep is good, you have the capacity to pursue what matters to you. You can train hard and see results. You can focus on demanding work. You can be patient with your children. You can be present with your partner. You can handle setbacks without falling apart. You can make choices that align with your values rather than defaulting to the path of least resistance. You can show up as the person you want to be.

When sleep is poor, that capacity diminishes. Not dramatically at first, and you can usually push through, compensate, and tough it out. But over time, chronic sleep deprivation erodes your capacity for living fully. You’re more reactive, more impulsive, less capable of self-regulation. You’re more at the mercy of circumstances, habits, and immediate impulses. Your agency (your ability to direct your life according to your values and goals) is compromised.

This is why sleep matters. Getting eight hours isn’t some achievement to tick off your optimisation checklist, you are working on this because adequate sleep maintains your capacity to live the life you want to live. It’s foundational in the truest sense; everything else is built on it.

Sleep as an Exercise in Agency

What sleep does is also provide a practice ground for exercising agency. Prioritising sleep requires making trade-offs. It means saying no to late-night activities sometimes. It means setting boundaries around your schedule. It means acknowledging that you can’t do everything and choosing to prioritise what matters most.

But more fundamentally, sleep confronts you with your radical freedom and the anxiety that accompanies it. Every evening, you face a choice: will you create the conditions for adequate sleep, or will you surrender to circumstances, distractions, or the expectations of others? There’s no external authority compelling you either way. The choice is yours, and with it comes responsibility.

The modern world is not set up to support good sleep. You have to actively create the conditions for it. This requires recognising where you’ve surrendered agency to circumstances, other people’s expectations, or the default patterns of contemporary life. Sartre would call this “bad faith”: the self-deception of pretending you have no choice when you actually do. When you say “I can’t get enough sleep”, examine that statement honestly. Often it means “I haven’t prioritised sleep” or “I’m afraid of what I’d have to give up to sleep adequately” or “I’m unwilling to disappoint others by setting boundaries” or “I’m avoiding the discomfort of acknowledging my constraints and making difficult choices”.

This isn’t to minimise real constraints. Some people face genuine obstacles to adequate sleep (shift workers, parents of young children, people with chronic pain or sleep disorders). But even here, Sartre’s distinction between facticity and freedom is illuminating. Your facticity (the brute facts of your situation) is given. You didn’t choose to be a shift worker or to have young children or to have a sleep disorder. But within that facticity, you still have freedom in how you respond.

Even within constraints, there’s usually more agency than we initially recognise. The question isn’t whether you can achieve perfect sleep, but whether you’re doing what’s within your control to make sleep as good as it can be within your circumstances. A shift worker can’t change their schedule, but they can control light exposure, sleep environment, and whether they prioritise sleep on days off. A parent of young children can’t change nighttime wakings, but they can examine whether they’re also staying up scrolling after the children are asleep, whether they’re saying yes to commitments they could decline, whether they’re asking for help when it’s available.

The crucial question: Are you making conscious choices within your facticity, or are you in bad faith, exaggerating your constraints to avoid the discomfort of acknowledging where you do have agency?

The discomfort of recognising your freedom around sleep reveals something important: you’re responsible for the life you’re building. If you consistently sacrifice sleep for work, you’re choosing work over health. If you sacrifice sleep for social media, you’re choosing distraction over capacity. If you sacrifice sleep to avoid disappointing others, you’re choosing their approval over your well-being. None of these choices are necessarily wrong, but they are choices. Owning them, rather than hiding behind “I can’t”, is the first step towards living authentically.

Sleep also confronts you with another existential reality: you are fundamentally an embodied being with physical needs, not a disembodied consciousness that can transcend bodily constraints through willpower. You cannot simply will yourself to function optimally without adequate sleep. You cannot overcome biological necessity through determination or discipline. Your body requires sleep, and ignoring this requirement has consequences. This is a useful corrective to the fantasy that you’re a purely rational agent who can override physical reality through force of will.

Heidegger’s notion of “thrownness” also applies here. You’re thrown into a body that requires sleep; you didn’t choose this constraint, but it’s an inescapable aspect of your existence. Yet you’re also thrown into possibilities. Within your embodied constraints, you can project yourself towards different futures. You can be the person who chronically undersleeps and lives diminished, or you can be the person who honours their embodied nature and maintains the capacity to live fully. Neither path is predetermined. You choose through your actions, and through those choices, you create yourself.

The anxiety that accompanies this freedom is real. Prioritising sleep means confronting what you’ll have to give up, what you’ll have to say no to, who you might disappoint. It means acknowledging that time is finite and you can’t do everything. It means accepting your limitations rather than fighting against them. This is uncomfortable, but it’s also the path to authenticity and living in accordance with reality rather than in denial of it.

What sleep does, then, is serve as a daily reminder that you are free, you are responsible, and you are creating yourself through your choices. Every night is an opportunity to exercise agency, to prioritise your capacity for living well, to choose who you’re becoming. Or it’s an opportunity to surrender that agency, to drift along with circumstances, to be in bad faith about your constraints and choices.

The question isn’t just “Am I getting enough sleep?” It’s “Am I taking responsibility for my life? Am I exercising my freedom? Am I living authentically or hiding from the anxiety of choice?” Sleep becomes a microcosm of how you approach your existence—with agency and authenticity, or with passivity and self-deception.

Sleep and the Good Life

Finally, understanding what sleep does should connect to a vision of human flourishing (eudaimonia). Health, including adequate sleep, is valuable not for its own sake but for what it enables. Sleep gives you the capacity to pursue excellence, to develop your capabilities, to contribute to others, to engage meaningfully with the world.

When you’re well-rested, you’re more capable of practical wisdom and making good judgments about how to live. You’re more capable of courage and doing what’s right even when it’s difficult. You’re more capable of temperance and actually exercising appropriate self-control. 

Sleep enables you to be the person you’re capable of becoming. It gives you the physical capacity to train and develop your body. It gives you the cognitive capacity to learn and develop your mind. It gives you the emotional capacity to develop meaningful relationships. It gives you the resilience to handle adversity and continue moving towards what matters.

This is why sleep isn’t just a health intervention, it’s a foundational practice for living well. It’s one of the most basic ways you take care of the instrument through which you engage with your life. Neglecting sleep is neglecting the very capacity that makes everything else possible.

Conclusion: What Does Sleep Do? 

We’ve covered considerable ground in exploring what sleep does. From hormonal regulation to metabolic health, from athletic performance to cognitive function, from body composition to stress resilience, sleep touches every aspect of how you function as a human being.

The key insights to take away are:

Sleep is an active process of repair, recovery, consolidation, and preparation for the future. It’s not passive downtime; it’s essential maintenance and development work.

What sleep does extends far beyond simply “feeling rested”. It affects every system in your body and every aspect of your mental function.

Sleep, diet, exercise, and stress management are deeply interconnected. Improving one makes the others easier; neglecting one undermines the others.

There’s significant individual variability in sleep needs and responses, but everyone needs adequate sleep for optimal function.

Acute sleep deprivation and chronic sleep restriction have different effects and different recovery timelines.

Sleep is foundational for human flourishing; it maintains the capacity to live fully, pursue what matters, and become who you’re capable of becoming.

Understanding what sleep does is the first step towards taking it seriously. The next articles in this series will explore how sleep works, how sleep is regulated, and ultimately how to consistently get good sleep. But the foundation is recognising that sleep isn’t optional, isn’t a luxury, and isn’t something you can compromise on indefinitely without consequences.

Sleep is one of the most fundamental ways you exercise agency over your life. It’s one of the clearest examples of how taking care of your body enables you to live more fully. It’s a practice in prioritisation, in making trade-offs consciously, in building the foundation that makes everything else possible.

You cannot be your best self without adequate sleep. You cannot live fully, pursue excellence, show up for others, or realise your potential whilst chronically sleep deprived. Sleep isn’t everything, but without it, everything is harder. Understanding what sleep does is understanding why it deserves your attention, your priority, and your commitment to doing what’s within your control to get it right.

As with everything, there is always more to learn, and we haven’t even begun to scratch the surface with all this stuff. However, if you are interested in staying up to date with all our content, we recommend subscribing to our newsletter and bookmarking our free content page. We do have a lot of content on sleep in our sleep hub.

If you would like more help with your training (or nutrition), we do also have online coaching spaces available.

We also recommend reading our foundational nutrition articles, along with our foundational articles on exercise and stress management, if you really want to learn more about how to optimise your lifestyle. If you want even more free information on sleep, you can follow us on Instagram, YouTube or listen to the podcast, where we discuss all the little intricacies of exercise.

Finally, if you want to learn how to coach nutrition, then consider our Nutrition Coach Certification course. We do also have an exercise program design course, if you are a coach who wants to learn more about effective program design and how to coach it. We do have other courses available too, notably as a sleep course. If you don’t understand something, or you just need clarification, you can always reach out to us on Instagram or via email.