Jet Lag Calculator

You’ve just landed in Tokyo, or New York, or Dubai, and the city is alive with everything you came to experience. But instead of being present for any of it, you’re staring at a hotel ceiling at 3am, wired and exhausted in equal measure, knowing that tomorrow you’ll drag yourself through meetings or sightseeing in a thick cognitive fog that no amount of coffee seems to cut through. By the time you finally start feeling normal, it’s almost time to fly home and do the whole thing in reverse.

This is the standard jet lag experience, and most people accept it as an unavoidable cost of long-distance travel. It isn’t. Jet lag is a circadian problem, and circadian problems have circadian solutions. The jet lag recovery and prevention tool above was built on decades of chronobiology research, and it generates a personalised protocol based on your actual biology rather than generic advice like “drink water and avoid alcohol.” Those things help, but they’re not what shifts your body clock.

What follows is the science behind every recommendation the tool makes. You don’t need to understand all of this to follow the protocol, but understanding why each intervention works means you can adapt intelligently when your plans inevitably collide with reality. And there’s something genuinely useful about knowing that when the tool tells you to seek bright light between 6am and noon, it’s not arbitrary. It’s based on where your core body temperature bottoms out and how your Phase Response Curve works.

What Jet Lag Actually Is 

Most people think of jet lag as tiredness from travel, but that’s only the surface. What’s actually happening is circadian desynchrony: a mismatch between your internal biological clock and the external environment you’ve landed in. Your body doesn’t just have one clock, either. It has a master clock in the brain called the suprachiasmatic nucleus, or SCN, and then dozens of peripheral clocks in organs like the liver, gut, heart, and muscles. Each of these runs on its own schedule, and they don’t all shift at the same speed when you cross time zones.

The SCN sits just above where your optic nerves cross and receives direct light input from specialised cells in your retina. This is why light is such a powerful signal for resetting your clock, something we’ll come back to repeatedly. Under normal conditions, the SCN coordinates all your peripheral clocks so that everything runs in sync: your temperature rhythm, your cortisol rhythm, your melatonin rhythm, your digestive enzymes, your immune function. When you fly across multiple time zones, the SCN starts receiving light signals that don’t match its current programme, and the whole system falls out of alignment.

Here’s the part that makes jet lag genuinely difficult. The human circadian clock doesn’t run on exactly 24 hours. Research by Charles Czeisler and colleagues at Harvard established that the average intrinsic period is approximately 24.18 hours, which means your body has a slight natural tendency to drift later each day. This has real consequences for jet lag, because it means your clock finds it easier to delay (shift later, as in westward travel) than to advance (shift earlier, as in eastward travel). Without any intervention, most people can shift about one hour per day going west, but only about 40 to 50 minutes per day going east. And that’s an average across healthy young adults. Older adults, people with irregular sleep schedules, and those who are already sleep-deprived will often adapt even more slowly.

The peripheral clock problem is what catches people off guard. Even after your brain clock has adjusted and you’re sleeping at reasonable hours, your gut and liver clocks can lag behind by several days. Research from Satchin Panda’s lab at the Salk Institute has shown that peripheral clocks can take up to six times longer to synchronise than the SCN. This is why many travellers report digestive issues, low energy, and impaired concentration that persists well after their sleep seems to have normalised. You’re not imagining it. Parts of your body are literally still on home time.

This is also why a single intervention, whether that’s melatonin or bright light or just willpower, is rarely sufficient for anything beyond a mild shift. You need a coordinated approach that targets the SCN through light and melatonin, and the peripheral clocks through meal timing and exercise, which is exactly what the tool is designed to provide.

How to Use the Jet Lag Recovery and Prevention Tool

The tool sits at the top of this page, and before we dive into the circadian science behind each recommendation, it’s worth walking through how to use it so you can generate your protocol and refer to it as you read.

The first thing the tool asks for is your mode: Pre-Trip Planning or Already There. These correspond to prevention and recovery. If you haven’t left yet, Pre-Trip Planning will build a protocol that begins shifting your schedule before departure so you arrive partially adapted. If you’re already at your destination, Already There generates a recovery protocol starting from the current day. Both modes use exactly the same circadian science, just applied to different starting points.

Next, you’ll enter your free-day bedtime and wake time. This is important, and the tool is specific about asking for your free-day times rather than your workday alarm. On days when you have no alarm and no obligations, when do you naturally fall asleep and wake up? These times determine your chronotype, and the tool uses them to estimate two critical circadian markers that anchor every recommendation in your protocol: your Core Body Temperature Minimum and your Dim-Light Melatonin Onset. We’ll explain exactly what these are and why they matter in the next section.

Then you’ll set your origin and destination timezones, along with your departure and arrival dates and times. The tool uses the timezone difference to calculate the direction and magnitude of your shift, and the departure time factors into in-flight recommendations.

Under Protocol Settings, you’ll find three options. Adaptation Speed lets you choose between Standard (one hour per day), Aggressive (1.5 hours per day), and Max-effort (two hours per day). The standard rate is well-supported in the literature and is what most people will achieve with good compliance. The aggressive rate is realistic if you follow all five intervention tracks consistently. Max-effort is based on rates achieved in controlled laboratory settings with six or more hours of daily 10,000-lux light exposure, and while it’s theoretically possible, very few people achieve it outside of a research lab. For most travellers, Aggressive is the sweet spot between ambition and reality.

Melatonin Dose offers a choice between 0.5mg (physiological) and 5mg (pharmacological). We’ll discuss why 0.5mg is the recommendation for most people shortly, but the key point is that more melatonin does not mean a bigger phase shift. The 5mg option is there for people who prefer the additional sedative effect, not because it shifts the clock faster.

Once you hit Generate My Protocol, the tool produces several outputs. At the top you’ll see your trip summary, severity score, and estimated adaptation timeline. Below that, the Daily Protocol tab shows a schedule overview table and expandable day cards with detailed five-track guidance: light, melatonin, caffeine, meals, and exercise, with specific times for each intervention. The Pre-Travel and In-Flight tabs provide context-specific advice, and the Forecast tab shows a chart of your projected adaptation curve compared to what would happen without intervention. You can also download everything as a PDF to take with you.

The day cards are where the real value lives. Each one shows your shifted sleep and wake targets for that day, along with precise windows for light seeking and light avoidance, melatonin timing relative to your DLMO, caffeine strategy with cutoff times, meal windows, and exercise timing. Everything shifts incrementally day by day as your protocol progresses.

Your Circadian Anchors: CBTmin and DLMO

Two biological markers underpin every timing recommendation in the tool, and understanding them will help you see why the specific windows matter so much.

Core Body Temperature Minimum (CBTmin) is the point in your 24-hour cycle when your core body temperature reaches its lowest. For most people, this occurs roughly two hours before their natural wake time, typically somewhere between 3am and 5am. You’ll never feel this happening. It’s not a conscious experience. But it’s the single most important reference point for light-based circadian interventions, because it sits at the pivot of your Phase Response Curve.

The Phase Response Curve, or PRC, describes how your clock responds to light depending on when you receive it. Light exposure in the hours before your CBTmin delays your clock, pushing it later. Light exposure in the hours after your CBTmin advances your clock, pulling it earlier. This is why the tool doesn’t just say “get morning light.” It calculates where your CBTmin falls and then determines a six-hour window on either side of it, telling you exactly when to seek light and when to avoid it based on whether you need to advance or delay your clock.

Dim-Light Melatonin Onset (DLMO) is when your body naturally begins producing melatonin in the evening, and it’s the anchor point for melatonin-based interventions. For most people, DLMO occurs roughly two hours before their natural sleep time. The tool estimates this from your free-day bedtime and then calculates when to take exogenous melatonin relative to DLMO to achieve maximum phase-shifting effect. Taking melatonin at a random time, which is what most travellers do, is far less effective than timing it precisely relative to this marker.

Your chronotype, which the tool calculates from the midpoint of your sleep period, determines where both of these markers fall on the 24-hour clock. An extreme night owl’s CBTmin might land at 7am, while an early bird’s might be at 3am. This is why two people on the same flight, crossing the same number of time zones, may need quite different light and melatonin schedules. The tool accounts for this automatically.

A brief note on accuracy: these estimates are based on population averages from research by Czeisler and colleagues. Individual CBTmin can range from one to three hours before natural wake time, and extreme chronotypes may deviate further. If you wear a device like an Oura ring, Apple Watch, or WHOOP that tracks overnight skin temperature, your temperature nadir closely approximates CBTmin and can be used to calibrate the tool’s recommendations. If the light and avoidance windows feel off after two or three days, try shifting them by an hour in either direction.

The Five Intervention Tracks

The tool generates recommendations across five distinct tracks, each targeting a different aspect of your circadian system. They work synergistically, and while any single track will help, the combined effect of all five is substantially greater than the sum of their parts. Think of them as five levers, each one pulling your body clock in the same direction.

Track 1: Light Exposure

Light is the most powerful zeitgeber, which is the German word chronobiologists use for “time giver,” and it’s the foundation of any serious jet lag protocol. Nothing else comes close in terms of raw phase-shifting power.

The mechanism is straightforward. Specialised retinal ganglion cells in your eyes, distinct from the rods and cones used for vision, are sensitive to blue-wavelength light and project directly to the SCN. When these cells detect bright light, they send a signal that resets the master clock. The direction of the reset depends entirely on timing relative to your CBTmin, following the Phase Response Curve described above.

For eastward travel, where you need to advance your clock (shift it earlier), the tool prescribes bright light in the six hours after your CBTmin. This is typically a morning light window, and it’s why getting outside early on arrival day is one of the highest-leverage things you can do. For westward travel, where you need to delay your clock (shift it later), the tool prescribes light in the six hours before CBTmin, which translates to an evening light window.

Intensity matters enormously. The threshold for meaningful circadian effects is around 2,500 lux, and the research on phase shifting typically uses 10,000 lux. To put that in perspective, a bright office is around 300 to 500 lux. Even a well-lit room rarely exceeds 500 lux. That’s nowhere near enough. Direct sunlight, even on a cloudy day, delivers around 10,000 lux. A dedicated light therapy box at 30 centimetres will deliver 10,000 lux. These are the intensities that actually move the needle, and they’re why the tool specifically recommends outdoor light or a light box rather than just “being in a bright room.”

Equally important is the avoid window. During the hours when light would shift your clock in the wrong direction, the tool recommends staying in dim light or wearing blue-light blocking glasses (amber-lensed). For shifts of eight hours or more, this becomes critical: light at the wrong time during the first 48 hours can actually reverse your adaptation, pushing your clock in the opposite direction and dramatically worsening jet lag. The tool flags this as the “Circadian Danger Zone” for large shifts.

Track 2: Melatonin

Melatonin is not a sleeping pill. This is the single most important thing to understand about how the tool uses it. Melatonin is a chronobiotic, meaning it shifts the timing of your biological clock, and its phase-shifting effects depend almost entirely on when you take it relative to your Dim-Light Melatonin Onset.

For eastward travel (phase advance), the tool recommends taking melatonin 30 minutes before DLMO. This is the timing that produces the strongest advance effect, pulling your clock earlier. The evidence for this comes from work by Alfred Lewy and colleagues, who mapped out the melatonin Phase Response Curve, and it’s been confirmed by the Herxheimer and Petrie Cochrane review and subsequent meta-analyses by Auld and Costello.

For westward travel (phase delay), melatonin’s role is different. Its phase-delay effect is relatively modest compared to its advance effect, so the tool recommends taking it about 60 minutes after DLMO, primarily for its sedative properties. It helps you fall asleep at the later target time, but the heavy lifting for westward adaptation comes from evening light exposure.

Now, about dosing: the tool defaults to 0.5mg, and there’s a good reason for this. Research consistently shows that 0.5mg, a physiological dose that roughly matches your body’s natural melatonin production, is just as effective as 3mg or 5mg for phase shifting. The Herxheimer and Petrie Cochrane review found no dose-response relationship for phase shifting above 0.5mg. The Auld et al. meta-analysis in 2017 confirmed this. Higher doses produce more sedation, which some people prefer, but they don’t shift the clock any faster.

This runs counter to what most people assume, and counter to what’s most commonly sold. Walk into any pharmacy and you’ll find melatonin in 3mg, 5mg, and even 10mg tablets. These pharmacological doses aren’t harmful for most adults, but they’re not doing what people think they’re doing. If you want a bigger phase shift, you don’t need a bigger dose. You need better timing. The 0.5mg option in the tool reflects the current scientific consensus: timing is everything, and dose beyond the physiological range is largely irrelevant for circadian purposes.

One important note: the tool’s melatonin recommendations are designed for adults aged 18 and over. If you’re taking prescription medications, particularly blood thinners, blood pressure medication, diabetes drugs, or immunosuppressants, consult your doctor before using melatonin. And melatonin dosing for children should always involve a paediatrician.

Track 3: Caffeine

Most travellers already use caffeine to manage jet lag, but they use it reactively, downing coffee whenever they feel tired. The tool takes a more strategic approach, treating caffeine as both an alertness tool and a mild chronobiotic.

Caffeine has a direct effect on the circadian clock, primarily through its action on adenosine receptors, and research has shown it can produce modest phase delays. This makes it particularly useful for westward travel, where you’re trying to push your clock later. The tool recommends strategic caffeine in the afternoon for westward trips, aligned with the time when its alertness effects and chronobiotic effects both work in your favour.

For eastward travel, the approach is reversed. Caffeine’s phase-delaying properties work against an advance goal, so the tool recommends using it only in the morning at destination time and cutting it off well before noon.

The cutoff window is calculated based on caffeine’s half-life, which runs between five and seven hours depending on individual metabolism. If your target bedtime is 11pm, the tool will set your caffeine cutoff at 5pm or earlier. This isn’t conservative for the sake of it. Caffeine consumed within six hours of bedtime has been shown to reduce total sleep time by over an hour and significantly impair sleep quality, even in people who claim they “sleep fine with caffeine.” They do sleep. They just don’t sleep as well as they would without it, which matters enormously when you’re trying to anchor a new circadian rhythm.

Track 4: Meal Timing

This is the track most people overlook entirely, and it may be the most important one for complete recovery. While light and melatonin primarily target the SCN (your master brain clock), meal timing is the dominant zeitgeber for peripheral clocks in the liver, gut, and other digestive organs.

Satchin Panda’s research at the Salk Institute has demonstrated that these peripheral clocks can lag behind the SCN by several days after time zone transitions. This is why you can feel like you’ve adjusted, sleeping and waking at reasonable hours, yet still experience digestive discomfort, poor appetite at mealtimes, unexpected energy crashes, and impaired glucose regulation. Your brain clock has shifted, but your gut is still running on home time.

The tool addresses this by calculating a 10-hour eating window anchored to local time, with your first meal 30 minutes after your target wake time and your last meal at least three hours before your target bedtime. It recommends eating on destination time from Day 1, even if you’re not hungry. The act of eating sends a powerful time signal to peripheral clocks through the food-entrainable oscillator, a secondary pacemaker that operates partly independently of the SCN.

The composition of your first meal matters too. A high-protein breakfast, think eggs, Greek yoghurt, fish, or meat, activates orexin and dopamine circuits that promote daytime alertness and help entrain the wake phase of your peripheral clocks. 

Track 5: Exercise

Exercise acts as a circadian signal through multiple pathways, including core body temperature elevation, cortisol release, and downstream effects on clock gene expression. Research by Youngstedt and colleagues mapped out an exercise Phase Response Curve showing that, similar to light, the timing of exercise relative to your circadian phase determines whether it advances or delays your clock.

For eastward travel, the tool recommends morning exercise within one to two hours of your target wake time. This amplifies the phase-advancing effect of morning light, and the two interventions work synergistically. A 20 to 30-minute outdoor jog or brisk walk on arrival morning delivers light exposure and exercise simultaneously, making it one of the most time-efficient interventions available.

For westward travel, the tool shifts exercise to the evening, aligned with the light-seeking window, to support phase delay. Evening exercise elevates core body temperature and delays the subsequent temperature drop that facilitates sleep onset, effectively pushing your sleep timing later.

The tool also flags an important constraint: avoid intense exercise within three hours of your target bedtime. High-intensity training elevates core body temperature and cortisol in ways that actively suppress melatonin onset and interfere with sleep initiation. Moderate aerobic exercise, 20 to 40 minutes, is the sweet spot. You want enough to generate a circadian signal without disrupting the sleep you’re trying to protect.

East vs. West: Why Direction Changes Everything

One of the first things you’ll notice in the tool’s output is whether your shift is classified as eastward (phase advance) or westward (phase delay), and this distinction shapes the entire protocol.

The reason direction matters comes back to the intrinsic circadian period. Because the human clock runs slightly longer than 24 hours (approximately 24.18 hours, per Czeisler’s 1999 research), your body has a natural bias towards delaying. This means westward travel, which requires you to push your clock later, works with your biology, while eastward travel, which requires you to pull your clock earlier, works against it.

In practical terms, this asymmetry means that a six-hour westward shift (say, London to New York) is meaningfully easier to recover from than a six-hour eastward shift (New York to London). Research by Waterhouse and colleagues in 2002 quantified this: eastward adaptation typically takes 30 to 50 percent longer than an equivalent westward shift. The tool accounts for this in its severity scoring and adaptation timeline.

For very large eastward shifts, the tool does something clever. When the eastward shift exceeds about nine hours, it calculates whether going “the long way round,” delaying westward instead of advancing eastward, would actually be faster. For example, if you’re flying 11 hours east, the tool might determine that a 13-hour westward delay (the remaining portion of the 24-hour clock) requires fewer days of adaptation. When it makes this recommendation, it flags it clearly in the output with a “Direction Optimised” alert.

The tool also flags what it calls the “Circadian Danger Zone” for shifts of eight hours or more. At these magnitudes, your CBTmin falls in a region of the Phase Response Curve where light exposure can unpredictably shift your clock in either direction. Getting light at the wrong time during the first 48 hours can reverse your progress and make jet lag dramatically worse. This is why the tool emphasises strict adherence to the seek and avoid windows during the early days of a large shift. When in doubt, it recommends wearing blue-light blocking glasses until your scheduled seek window opens.

Prevention vs. Recovery: Two Modes, One Goal

The tool offers two distinct modes, and choosing the right one depends on where you are in relation to your trip.

Pre-Trip Planning mode is designed for people who haven’t left yet and want to arrive partially adapted. It builds a protocol that begins shifting your schedule in the days before departure, typically recommending adjustments of 30 to 60 minutes per day in the direction of travel. Research by Eastman and colleagues in 2005 demonstrated that pre-flight circadian shifting reduced post-flight jet lag severity by approximately 40 percent, which is a substantial improvement for what amounts to a few days of slightly altered sleep and light timing.

The tool caps the pre-departure protocol at seven days, which is practical for most trips. Even pre-shifting by two or three hours before a large eastward journey can make a noticeable difference in how you feel on arrival. The Daily Protocol tab shows this as a series of day cards counting up to Day 0 (departure), with each card showing your incrementally shifted targets for sleep, light, melatonin, meals, and exercise.

It’s worth noting that the tool also flags when pre-shift schedules might conflict with normal life. If your eastward protocol requires going to bed at 8pm or waking at 3am several days before departure, a schedule warning appears suggesting you do as much as your commitments allow and accelerate the remaining shift on departure day or upon arrival. This is realistic rather than idealistic. Partial pre-shifting still provides meaningful benefit even if you can’t hit every target perfectly.

Already There mode generates a recovery protocol that starts from the assumption you’re at your destination with your clock still running on home time. The structure is the same: day cards with five-track interventions, shifting incrementally towards full adaptation. Day 1 is arrival day, and the protocol runs until the tool projects you’ll be fully synchronised with local time.

Both modes use identical circadian science and the same five intervention tracks. The difference is simply the starting point and the framing. Prevention reduces the debt you accumulate on arrival. Recovery pays it off after the fact. Either way, a structured protocol significantly outperforms the “just push through and hope for the best” approach that most travellers default to.

When the Plan Meets Reality

No protocol survives first contact with a red-eye flight, a delayed connection, or a dinner invitation that runs past your target bedtime. The value of understanding the science behind the tool is that it allows you to make intelligent compromises when life doesn’t cooperate.

The first thing to know is the hierarchy of interventions. If you can only do one thing, make it light management. This is the single most powerful lever for shifting the SCN, and the research is unambiguous on its primacy. If you can do two things, add melatonin timing, especially for eastward travel where its phase-advance effect is strongest. Meal timing comes next for its peripheral clock effects, followed by exercise and caffeine. When circumstances force you to drop parts of the protocol, drop from the bottom up.

Imperfect compliance still beats no protocol at all. If you hit 70 percent of your targets across all five tracks, you’ll adapt meaningfully faster than someone who does nothing. The tool provides the ideal schedule, and reality provides the constraints. Your job is to close the gap as much as reasonably possible.

For short trips, the tool’s approach may need modification. If your stay at the destination is shorter than the projected adaptation time, full adaptation means you’ll face reverse jet lag on the way home. In these cases, partial adaptation of 50 to 70 percent, combined with strategic light management and napping, may be more practical than trying to fully entrain and then re-entrain on return. The tool flags this with an alert when it detects a large shift that would require more days than a typical short trip allows.

The tool adjusts for adults over 65, who typically adapt at roughly half the rate of younger adults due to reduced retinal light sensitivity, lower melatonin amplitude, and weaker circadian signalling. If you’re in this group, the projected timeline will be longer, and patience with the process becomes more important. Consulting a doctor about melatonin use is also recommended for older adults, particularly those on multiple medications.

Finally, there are situations where professional guidance is warranted. For shifts over eight hours with recurring travel, for people with existing sleep disorders, or for those on medications that interact with melatonin or affect circadian function, a sleep specialist can offer interventions beyond what any self-service tool can provide. The tool is built for healthy adults making occasional or moderate-frequency long-haul trips. It’s a powerful starting point, not a substitute for medical advice when the situation calls for it.

Jet Lag Recovery and Prevention Tool

The reason any of this matters extends beyond the mechanics of sleep and body clocks. You travel for a reason, whether that’s work that demands your sharpest thinking, a holiday where you want to be present for every moment, or time with people you don’t see often enough. Spending the first three days of a week-long trip in a haze isn’t just uncomfortable. It’s a waste of the very thing you travelled for.

The jet lag recovery and prevention tool gives you a protocol. The science in this article hopefully gives you the understanding to use it well. Between the two, you have everything you need to arrive somewhere brilliant and actually be there for it, clear-headed, energised, and ready to engage fully with whatever brought you across those time zones in the first place.

Generate your protocol above, save the PDF, and follow it. Your body clock is remarkably responsive to the right signals at the right times. You just need to know which levers to pull and when to pull them.

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.

This article and tool was created by Paddy Farrell.