Jet Lag Calculator
Jet lag is a mismatch between your body clock and the clock where you land. This calculator measures that mismatch in hours for your exact flight, then returns a dated schedule of sleep, bright light and darkness that closes it. Enter your route, your flight times and the hours you usually sleep.
- 3,244 airports
- Daylight saving aware
- Runs in your browser
- PDF you can take with you
What the Jet Lag Calculator gives you
The tool returns a dated schedule, a severity reading and a recovery estimate. Every number it shows is derived from your own route and your own sleep hours.
A dated schedule, not general advice
The plan names every date from your first preparation day to the day your body clock matches local time. Each date carries a sleep window, a bright light window and a low light window, and the whole schedule downloads as a PDF.
Light windows anchored to CBTmin
Your circadian temperature minimum sits about 2 hours before you normally wake. The calculator tracks that anchor as it migrates and places every light window on the correct side of it.
Direction aware arithmetic
Eastward flights advance the clock at 1 hour per day. Westward flights delay it at 1.5 hours per day. Eastward jumps of 9 hours or more reverse into delays, because the long way round costs fewer days.
Real zones and real daylight saving
Every one of 3,244 airports carries an IANA time zone identifier. Offsets resolve at the instant you depart and the instant you land, including half hour zones such as India and quarter hour zones such as Nepal.
Melatonin and caffeine timing
The plan marks the window where a low melatonin dose pushes the clock the right way, and the point after which caffeine sits in your blood through the sleep you are protecting.
Everything runs on your device
The calculation happens in your browser. Your route, your flight times and your sleep window never reach a server, and no account is required to use the tool.
What the Jet Lag Calculator measures
Four variables decide the whole schedule. Change any one of them and every window in the plan moves.
- Variable 1
Time zone shift
Destination offset minus origin offset, in hours
The shift is the size of the mismatch your body clock has to close. It is resolved from IANA zone identifiers at the exact instant of departure and the exact instant of landing, so a flight across a daylight saving change is measured correctly.
- Variable 2
Direction of travel
Eastward advances the clock, westward delays it
Direction decides which way light has to move your clock and how fast it can travel. Advancing runs against the body's natural drift at 1 hour per day. Delaying runs with it at 1.5 hours per day.
- Variable 3
Your habitual sleep window
Bedtime and wake time set your CBTmin
Your circadian temperature minimum sits 2 hours before habitual wake. Light reaching your eyes after that point pulls the clock earlier. Light before it pushes the clock later. Without your sleep hours, no light window can be placed correctly.
- Variable 4
Preparation days
Zero to four days before departure
Preparation days bank part of the shift at home, where the bedroom and the routine are already familiar. Each day banked is one day removed from the recovery at the far end.
How the Jet Lag Calculator builds your plan
The calculator needs four facts about your trip. It returns a schedule in under a minute.
- 1
Enter your route
Type a city or a three letter IATA code at both ends. The calculator reads the IANA time zone attached to each airport.
- 2
Enter your flight and sleep times
Give the local departure time, the local landing time, and the hours you normally sleep at home.
- 3
The calculator places your CBTmin
It locates your circadian temperature minimum, measures the shift your body has to make, and picks the cheaper direction.
- 4
You receive a dated schedule
Sleep, light, dark and melatonin windows land on named dates. Download the plan as a PDF or keep it open on your phone.
The full method, including the light timing rules and the shift rates, is set out in how the calculator works.
Jet lag east against jet lag west
Direction of travel is the single attribute that changes the most. The same number of zones costs more days eastward, and every instruction in the plan reverses.
| Attribute | Flying east | Flying west |
|---|---|---|
| What the body clock must do | Advance, run earlier | Delay, run later |
| Relation to natural drift | Against it | With it |
| Shift rate per day | 1 hour | 1.5 hours |
| Seek bright light | Four hours after target wake | Four hours before target bed |
| Keep light low | Three hours before target bed | Three hours after target wake |
| Melatonin timing | Five hours before target bed | On waking |
| Unassisted recovery | About one day per zone | About two thirds of a day per zone |
| Typical symptom | Cannot fall asleep, alarm arrives early | Sleep arrives early, breaks at 3am |
The asymmetry has one cause. An unentrained human body clock runs slightly longer than 24 hours, so it drifts later on its own. Westward travel asks for a later clock and moves with that drift. Eastward travel asks for an earlier clock and moves against it. Past 9 hours east the calculator reverses direction and delays the clock the long way round instead, because a circle reaches the same point either way.
Time zones crossed on common flight routes
Twelve long haul routes, measured by the calculator itself. The last two columns are what a plan is worth on each one.
| Route | Shift | Zones | Direction | Body clock | Severity | With a plan | Without |
|---|---|---|---|---|---|---|---|
| Sydney to Los Angeles | -19h | 19 | West | Delay | Severe | 10 | 11 |
| London to Sydney | +11h | 11 | East | Delay | Severe | 6 | 11 |
| London to Tokyo | +9h | 9 | East | Delay | Severe | 7 | 9 |
| San Francisco to Paris | +9h | 9 | East | Delay | Severe | 7 | 9 |
| Dubai to New York | -9h | 9 | West | Delay | Significant | 3 | 5 |
| London to Singapore | +8h | 8 | East | Advance | Severe | 5 | 8 |
| Los Angeles to London | +8h | 8 | East | Advance | Severe | 5 | 8 |
| London to Los Angeles | -8h | 8 | West | Delay | Significant | 3 | 5 |
| London to New York | -5h | 5 | West | Delay | Moderate | 1 | 3 |
| New York to London | +5h | 5 | East | Advance | Significant | 2 | 5 |
| London to Dubai | +4h | 4 | East | Advance | Moderate | 1 | 4 |
| New York to Los Angeles | -3h | 3 | West | Delay | Moderate | 0 | 2 |
Offsets resolve for mid-January, when most of the northern hemisphere sits on standard time. Daylight saving moves several of these routes by one hour across the year, which the calculator resolves against your own dates. Figures assume a 23:00 to 07:00 sleep window and three preparation days.
Who uses a jet lag plan
Anyone crossing three or more time zones carries a body clock that no longer matches local time. These four groups pay the highest price for that gap.
Long haul business travel
A meeting on the morning after landing leaves no room for a slow recovery. Banking the shift before departure puts you in the room already aligned.
Holiday travel
Jet lag takes the first days of a trip, which are the days you paid for. A two day head start returns most of them.
Cabin crew and frequent flyers
Repeated crossings stack circadian debt. A plan per rotation keeps the body clock anchored rather than drifting between zones.
Athletes and performers
Reaction time, grip strength and peak power all track the body clock. Aligning the clock to local event time protects the performance window.
Jet Lag Calculator questions
Answers to what travellers ask before they build a plan.
What does the Jet Lag Calculator do?
The Jet Lag Calculator measures the gap between your body clock and the clock at your destination, then returns a dated schedule that closes it. The schedule names a sleep window, a bright light window and a low light window for every day from your first preparation day to the day your body clock matches local time.
How many time zones cause jet lag?
Jet lag begins at about two zones and becomes reliable at three or more. Below one hour of difference the body clock does not need to move, and the calculator says so instead of inventing a schedule. Severity climbs with each zone crossed and climbs faster eastward than westward.
Why is flying east harder than flying west?
The human body clock runs slightly longer than 24 hours. Delaying it, which is what westward travel asks for, works with that natural drift at about 1.5 hours per day. Advancing it, which is what eastward travel asks for, works against it at about one hour per day. Five zones east therefore costs more days than five zones west.
What is the circadian temperature minimum?
The circadian temperature minimum, written CBTmin, is the point in the daily cycle where core body temperature and alertness bottom out and the body clock is most sensitive to light. It sits about two hours before habitual wake time. Light after CBTmin advances the clock. Light before it delays the clock. Every window the calculator emits is positioned against that point.
How far before my flight should I start the plan?
Three days before departure is the practical sweet spot. Each preparation day moves your clock one hour when advancing and 1.5 hours when delaying, and those hours are banked at home where your sleep environment is already familiar. The calculator still builds a working plan if you start on the day you fly.
When should I seek light and when should I avoid it?
Travelling east, seek bright light in the four hours after your target wake time and keep light low in the three hours before your target bedtime. Travelling west, reverse both: seek bright light in the four hours before bed and keep it low in the three hours after waking. The calculator moves both windows each day as your body clock migrates.
Does the calculator handle daylight saving time?
Yes. Every airport carries a real IANA time zone identifier, and the calculator resolves the offset for the exact instant you depart and the exact instant you land. A flight that crosses a clock change is measured correctly, including half hour zones such as India and quarter hour zones such as Nepal and the Chatham Islands.
Is jet lag the same as travel fatigue?
No. Travel fatigue comes from the journey itself: dehydration, cabin pressure, cramped seating and lost sleep. It clears after one good night. Jet lag comes from a body clock that no longer matches local time, and it persists until the clock realigns. A long north to south flight produces travel fatigue and no jet lag, because it crosses no time zones.
Is my flight information stored anywhere?
No. The calculation runs entirely in your browser. Your route, your flight times and your sleep window are never sent to a server, never written to a database and never shared with a third party.
Is the Jet Lag Calculator free?
Yes. The calculator is free, it requires no account, and it places no limit on the number of plans you build. Each plan downloads as a PDF you can print or carry on your phone.
Your next flight already has a plan waiting
Three preparation days remove most of the cost of a long crossing. Build the schedule now and start it before you pack.
Build my jet lag plan