Jet lag calculator: how many days it takes, and what to do on each of them

Short answer

Your body clock shifts roughly 1 hour a day flying east and 1.5 hours a day flying west, so six zones east takes about six days and six west about four. Direction matters more than distance. The table below gives your number, and the schedules underneath give the light and sleep timing that actually produces it.

If you fall apart on day three and conclude you are handling this badly, you are not imagining the difficulty and it is not your fault. Jet lag is a physical mismatch between your body clock and the clock on the wall, it happens to almost everyone who crosses several time zones, and the rate at which it resolves is close to fixed — about an hour a day — no matter how disciplined you are about it. Nothing you can buy speeds it up much. What you can change is the timing of light and sleep, and that is what the rest of this page is.

Note: this article explains what travel-medicine guidance and circadian research say, and is not medical advice. Melatonin is a supplement, not an approved treatment for jet lag, and the timing matters more than the dose — talk to a clinician before using it, particularly if you are pregnant, taking other medication, or managing epilepsy or a clotting disorder.

How long does jet lag last? The numbers for your flight

Every jet lag calculator is built on the same two rates. The CDC's travel health guidance puts circadian adaptation at roughly 1 hour per day travelling east and 1.5 hours per day travelling west (1). Studies that followed real travellers landed in the same place: about 57 minutes a day after eastward flights and about 92 minutes a day after westward ones (2).

That is the whole calculation. What follows is it, done for you.

Two things to read carefully before you use your number.

It is an average, not a promise. The same flight produces a two-day recovery in one person and a week in another. Age, how much sleep you lost getting to the airport, how much daylight you actually get on arrival, and whether you are an early bird or a night owl all move it (2, 360529-7)).

Past about eight zones east, the arithmetic changes shape. At that distance the clock frequently re-entrains in the wrong direction on its own — drifting later rather than earlier, which researchers call antidromic re-entrainment — so a plan that insists on advancing tends to lose (2). The better move is to send your clock the long way round the dial: later each day, until it comes round to local time from the other side. London to Tokyo is nine zones east, which means a fifteen-hour delay rather than a nine-hour advance.

Is jet lag worse going east or west?

East, for almost everybody, and the reason is structural rather than psychological.

Left alone in constant conditions, the human circadian clock runs slightly longer than 24 hours. Every day you reset it by a few minutes, mostly with morning light. That means your clock is already built to be pushed later easily and pulled earlier with difficulty — a plane just makes you do a week of it at once (360529-7)).

Flying west asks you to stay up later and sleep in, which is the direction your clock drifts anyway. Flying east asks you to fall asleep when your body thinks it is late afternoon and get up when it thinks it is the middle of the night. That is the version people describe as feeling physically wrong rather than merely tired.

Our own explainer on how jet lag works goes through the mechanism in more detail. For the calculator, the practical consequence is the one in the table: the same six time zones cost you about six days eastbound and about four westbound.

Where the numbers actually come from: your body-clock low point

Every instruction in the schedules below is positioned relative to one moment in your day.

Core body temperature reaches its minimum — researchers write it CBTmin — roughly three hours before your usual wake time. If you normally get up at 07:00, yours is near 04:00 (2).

That point is a hinge. Bright light after it pulls your clock earlier. Bright light before it pushes your clock later (4). The same walk outside advances your clock or delays it depending on which side of that line it falls — which is why "get plenty of daylight" is not advice, and why two people following the same vague tip can end up moving in opposite directions.

This is also why the plans below spend as much attention on avoiding light as on seeking it. An hour of morning sun on a westward trip can undo a whole evening's work.

What to do flying east: the advance schedule

You are pulling your clock earlier, about an hour a day. Light goes in the morning; darkness goes in the evening.

• 3 days before — Move bedtime and wake time 1 hour earlier each day (half an hour if mornings are genuinely hard for you). Get bright light as soon as you wake on the new schedule.

• Evenings before — Dim the last 3 hours before your new bedtime. Screens down, lamps low. Light now cancels the morning's work.

• On the plane — Set your watch to destination time at boarding and live by it from that moment. Sleep only if it is night where you are going.

• Day 1 after landing — Get outside within the first hour of waking, and stay out for 2–3 hours if you can. Hold out for local bedtime even if the evening feels endless.

• Days 2 onward — Same fixed local wake time, same morning light, same dim evening. Repeat until the table's day count runs out.

• Optional — 0.5 mg melatonin about 4.5 hours before your target sleep onset — earlier in the evening than most people take it, and that timing is the point (5).

Two details that decide whether this works.

Outdoors, not a window. Measured illuminance outdoors runs into the thousands of lux; typical indoor lighting is a couple of hundred. A bright room is not a substitute for a street.

The evening half is not optional. People do the morning walk, then spend the evening under full house lights and a laptop, and wonder why day four looks like day one. Advancing a clock requires both halves.

What to do flying west: the delay schedule

You are holding your clock later, about an hour and a half a day. Light goes in the evening; sunglasses go on in the morning.

• 2–3 days before — Move bedtime and wake time 1–1.5 hours later each day, as far as your schedule allows.

• Evenings before — Bright light for the last 2–3 hours before your new bedtime. Stay up, stay lit.

• On the plane — Set your watch to destination time at boarding. Stay awake if it is daytime where you are going, however late it feels.

• Day 1 after landing — Dark sunglasses outdoors in the early morning, until about an hour after you get up. Save your daylight for the late afternoon and evening.

• Days 2 onward — Same pattern: protected mornings, bright evenings, bedtime held as late as the plan needs.

• Melatonin — Not part of this plan. The evidence for melatonin is in advancing the clock; for delays, light does the work (2).

The counterintuitive instruction is the sunglasses. On a westward trip, early morning light lands after your body-clock low point and pulls your clock in exactly the direction you are trying to move away from. Most people's instinct — throw open the curtains, go for a run — is the single most effective way to prolong a westward adjustment.

When should I start shifting before the flight?

Earlier than almost anyone does, and it pays disproportionately.

In a controlled study of advancing circadian rhythms before eastward travel, three days of a progressively earlier schedule combined with morning bright light moved participants' clocks by more than an hour a day, before they ever left home (6). Each pre-flight day is a day you do not spend wrecked at your destination.

Even one or two days helps. If you can only manage one, make it the day before, and make it a real shift of the whole schedule — bedtime, wake time and light — not just an early alarm.

Do not try to do it all at once. Advancing more than about an hour a day usually fails; you lie awake at the new bedtime, lose sleep, and arrive worse off than if you had done nothing.

Should I shift at all for a short trip?

Often no, and this is the question jet lag calculators are worst at.

For a trip of two nights or fewer across several time zones, shifting your clock takes longer than the trip, and then you have to shift it back. Travel-medicine guidance for short trips is to stay on home time where the schedule allows it: keep meals, meetings and sleep as close to your home clock as you can, and treat local time as a formality (1).

The practical version: book the meeting for what is late morning at home, eat when you would eat at home, and accept that you will be out of step with the city. You will function better in the hours that matter and come home without a second adjustment to make.

Does melatonin actually help with jet lag?

It has the best evidence of anything you can buy, and the worst record of being used correctly.

Clinical practice parameters from the American Academy of Sleep Medicine recommend timed melatonin for jet lag (7). Controlled work on dosing found that 0.5 mg taken at the right time advanced the clock about as much as 3.0 mg, with less daytime sleepiness (5). More is not better.

What matters is the hour. Melatonin has a phase response curve of its own, roughly the mirror image of light's: taken in the early evening it advances your clock, taken in the morning it delays it (8). Taken at the wrong biological time, it moves you the wrong way — which is the most common thing that happens when people take it at bedtime on arrival without working out where their own clock currently sits.

Three caveats worth saying plainly. In the US it is sold as a dietary supplement and is not FDA-approved for jet lag, so product content is not guaranteed to match the label. It makes some people sleepy at an inconvenient hour. And it is a conversation to have with a clinician before a trip, not a decision to make in an airport pharmacy — especially if you are pregnant, on other medication, or managing epilepsy or a clotting disorder. Our piece on melatonin timing versus dose goes further into why the hour beats the milligrams.

Why do jet lag calculators give different answers?

Because they are not all answering the same question, and most do not say which one they are answering.

Some give recovery time; some give a schedule. "Six days" and "wake at 05:30 on Tuesday" are different outputs from the same two rates.

Some assume you start on arrival; some assume you prepare. Three pre-flight days change the answer by three days.

Some use the travel-medicine rates; some use the laboratory ones. Under optimally timed bright light in a lab, clocks shift faster than they do in a hotel where light exposure is whatever the week hands you (2).

Few of them handle the eight-zone reversal. A calculator that tells you to advance nine hours for a London–Tokyo flight is doing arithmetic that the body does not cooperate with.

And none of them know your chronotype. Night owls advance more slowly than the average and delay more easily; early birds are the reverse. The published rates are population averages, and you are one person.

That last point is the useful one. The table gives you a starting estimate. What tells you whether it fits you is watching what your own body does over the first few days — which is the next section.

You are not weak on day three — your clock moves about an hour a day

This is the part people get wrong about themselves.

Somewhere around the third or fourth day, with a clear head in the morning and a wall at 3pm, most people conclude they are handling it badly. They push: a run, more coffee, a full schedule, an early night that doesn't happen. The thinking is that jet lag is a willpower problem with a motivation solution.

It is not. The rate is close to fixed. An hour a day eastbound is not a target to beat with discipline — it is roughly what a human circadian system does when you give it well-timed light, and nothing you can buy speeds it up much. The levers are light, timing and sleep opportunity, and that is the list.

What changes the experience is having a shape for it: knowing that day three of six is day three of six. Struggling on it is not weakness and it is not your fault — it is the rate. Before you blame yourself, look at what your body already recorded — the trend over the first week usually tells a calmer story than the afternoon does.

Living with jet lag: the questions people ask next

Should I exercise on the first day?

Light exercise outdoors is useful mostly because of the light, not the exercise. A walk at the right hour does more for your clock than a hard session at the wrong one. Save anything demanding for day three or four — your resting heart rate is usually still elevated before that, and training into a body that has not slept properly mostly buys you a worse night.

Does alcohol on the flight make jet lag worse?

It makes the sleep worse, which makes the jet lag feel worse. Alcohol shortens the time to fall asleep and then fragments the second half of the night, exactly when you are already fighting your clock. It also does nothing to your circadian phase, so you get the cost without the shift. Our piece on alcohol, next-day heart rate and sleep has the detail.

Can I just not sleep on arrival and force the reset?

Staying up through the first local day does push you toward local bedtime, and plenty of travellers swear by it. The cost is a large sleep debt layered on top of a clock shift, which tends to show up on days two and three. It is a reasonable tactic for a one- or two-zone hop, and a rough one past five.

What about coffee?

Caffeine holds you up through the destination daytime, which is genuinely useful when the plan asks you to stay awake. Its half-life is around five hours, so an afternoon coffee is still working at bedtime — on an eastward trip, where the whole job is falling asleep earlier, that is the thing most likely to undo the day. See caffeine, wired and tired.

Does jet lag get worse as you get older?

Many people report that it does, and the usual explanation is that sleep becomes lighter and more easily disrupted with age rather than that the clock itself shifts more slowly. There is no age at which the published adaptation rates stop applying.

I fly short-haul constantly and feel like this without crossing zones. Is that jet lag?

Not in the circadian sense, but something related. A sleep schedule that swings by several hours between workdays and days off produces a similar mismatch without a plane — researchers call it social jet lag, and it has been linked with higher body mass index independently of how long people sleep (10). The fix is the same shape: consistent timing and morning light.

Which doctor do I even ask about this?

For a one-off trip, your GP or a travel clinic. For sleep that has not settled weeks after a trip, or for a job that crosses zones continually, a sleep specialist. Bring dates: when you flew, which direction, how many zones, and what your sleep has actually been doing since.

When jet lag is worth a doctor's visit

Most jet lag resolves on the schedule in the table. A few situations are different.

It outlasts the arithmetic by a wide margin. Sleep that has not settled two or three weeks after a trip is no longer jet lag running its course, and is worth a conversation — particularly if it looks like insomnia that has taken hold on its own.

You cross time zones constantly for work. Repeated, unresolved circadian disruption is a different problem from a single trip and has its own management.

You have a condition the schedule interacts with. Epilepsy, diabetes on a fixed insulin schedule, a clotting disorder, a psychiatric condition with a sleep component, or any medication timed to your day — all of these need planning before departure rather than improvisation on arrival.

Something is happening that is not a clock problem. Chest pain, breathlessness, or a hot, swollen, painful calf after a long flight is not jet lag and needs urgent assessment. Severe headache, fainting, or confusion is the same.

You are pregnant, or travelling with a very young child. Worth asking what to adjust and what to leave alone before you go.

How to bring this up with your doctor

What to ask for, by name. If you want to use melatonin, ask about timed low-dose melatonin for jet lag, and say the direction and number of time zones. The hour is the prescription; the dose is secondary.

What to bring. Your usual bedtime and wake time, the flight times, how long you are away, and any medication with a timing requirement.

What to say, out loud. "I'm flying [N] zones [east/west] on [date] for [N] nights. What should I do about my [medication / insulin / anticoagulant] timing, and is timed melatonin reasonable for me?"

What to ask next. "Should I shift before I go, or stay on home time?" · "Which of my medications are tied to a clock rather than a meal?" · "What would make this worth a call while I'm away?"

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How Welltory helps — and what it cannot do

The limits first, in plain terms. Welltory is a general wellness product, not a medical device. It does not diagnose, detect, predict, monitor, prevent, treat or mitigate any condition. It does not measure your circadian phase. Nothing on your wrist knows where your body-clock low point actually sits; that is measured in a lab from melatonin or core temperature, and the three-hours-before-waking figure is an estimate. The app's measurements may be less accurate for people under 18 and during pregnancy. If a number in Welltory and a symptom disagree, the symptom goes to a clinician.

What it can do is tell you whether the plan is working. The table predicts about an hour a day. A dated record of your own sleep, resting heart rate, heart rate variability (HRV) and stress minutes shows you what actually happened — and the thing to watch is when you fell asleep, not how long you slept. If your sleep onset is creeping toward local time at roughly the rate the table predicts, the schedule is doing its job, whatever the afternoon feels like.

Resting heart rate usually settles last. It tends to sit a few beats above your own normal for as long as the mismatch lasts, including on days you feel fine. When it comes back into your usual range, the clock has largely caught up. Resting heart rate and overnight HRV are recorded the same way every day, which is what makes them comparable to your own history rather than to anyone else's.

You note what happened; the app records the rest. Add a tag from the plus sign at the top of the feed — "landed," "slept 4h," "took melatonin," "first full night." When the Today screen marks a stress or rest stretch, it asks "What happened?"; tap a suggested tag, type, or talk, and fix the tags before saving.

Over a couple of months, My Patterns shows what your tags have in common. My Patterns collects tags from stress and rest stretches (iOS with an Apple Watch or Oura) and needs roughly seven tagged events in a month before patterns appear, with history from the month before. One trip will not produce one; four trips a year might.

A record you made, for the next trip. The Journal (Premium) keeps your measurements, tags and notes together; from the web app you can export a CSV (Dashboard → choose a chart → Export). Sleep analysis shows how your heart rate behaved overnight, and Battery and stress minutes need iOS with an Apple Watch or Oura. Two or three logged trips tell you your own rate, which is more useful than the population average you started with. Whatever shows up there is an association to discuss with your doctor, not a cause.

The red flags stay the same size. A hot, swollen, painful calf, chest pain or breathlessness after a long flight needs urgent medical attention, whatever your app shows.

How we made it

Made with AI tools, then edited and fact-checked by the Welltory team. See our Editorial & AI policy.

Jane Smorodnikova, co-founder of Welltory, built the methodology behind how we read physiological data.

Written by Tatsiana Yashyna.

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