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Jet Lag Calculator: Light and Caffeine Windows
Two timezones and two clock times in, an adjustment window and an arrival day light plan out. It runs in your browser, it stores nothing, and every figure under it names the study it came from.
Used to place your body clock minimum, which sits roughly two hours before it.
Used only for the caffeine cutoff.
These are standard offsets. Daylight saving moves several of them by an hour, so pick the offset that will be in force on your travel dates.
Arrival day
| Estimated body clock minimum | 11:00 am |
|---|---|
| Seek bright light | 11:00 am to 2:00 pm |
| Keep light low | 8:00 am to 11:00 am |
| Last 107 mg coffee | 2:12 pm |
All times are destination local time on arrival day.
On a shift this large the hours to keep light low land in the destination morning, which is the opposite of the usual advice to get out in the morning sun. That is not a slip. Until your clock has moved, morning light at the destination still arrives before your body clock minimum, where it pushes the clock later and away from where you want it.
Caffeine is an alertness window, not a clock reset. It can hold you awake through a destination afternoon that your body reads as the middle of the night, and it does nothing at all to move the underlying rhythm. The light windows are the part with the mechanism behind them.
What this calculator does
Four inputs, and everything after them is arithmetic. It counts the zones and the direction, estimates how long adjustment takes, places your body clock minimum in destination local time, brackets the hours around it where light helps and where it hurts, and subtracts a published caffeine cutoff from your bedtime. Direction is derived, not asked: past twelve zones the change is smaller measured the other way round, and the smaller of the two is what the body responds to.
Why eastward is the harder direction
With no time cues to correct it, the human clock does not run at exactly 24 hours. Duffy 2011 (n=157 adults, 52 women and 105 men, aged 18 to 74) estimated intrinsic period in a month-long inpatient protocol and put the average at 24.15 hours, roughly nine minutes long. A clock that drifts later on its own is easier to push later still, which is what flying west asks for. Flying east asks for an advance, against the grain, so the same number of zones costs more.
That paper is also the reason to distrust a single confident number. Women in it averaged 24.09 hours and men 24.19, and 35 percent of the women and 14 percent of the men ran shorter than 24 hours outright. For those people the clock drifts earlier and the east-is-harder asymmetry weakens or disappears.
The adjustment window is a rule of thumb, not a measurement
The window is built from the rates usually quoted in the jet lag literature: the clock advances by roughly an hour a day and delays by roughly one and a half to two. That is about two thirds to one full day per zone eastward, and a half to two thirds of a day per zone westward. These are conventions distilled from field observations of travellers and aircrew, not constants anyone measured, and the spread between people is wide. Read it as an order of magnitude: four to six days means most of a week, not one good night.
Light is the lever with the mechanism behind it
The reference point is the core body temperature minimum, the low point of the daily rhythm, which sits roughly two hours before habitual wake time. Light after that point shifts the clock earlier, an advance. Light before it shifts the clock later, a delay. The effect changes sign at the minimum, which is why the same walk outside can help or hurt depending only on the hour you take it.
The calculator estimates that minimum from your home wake time and moves it into destination local time. On arrival day your clock has not shifted yet, so it is honest for day one and progressively wrong after it: the pattern slides by roughly the adjustment rate above as you adapt. The three hour brackets are practical handles, not physiological edges. The response curve is continuous, strongest near the minimum and tapering either side, so the usable version is getting outdoors inside one window and staying dim inside the other.
One consequence catches people out. After a large eastward shift the destination morning still falls before your body clock minimum, so morning light delays you, the wrong way. That is why the tool sometimes says keep the morning dim and go out in the afternoon. The usual advice to chase the sunrise is not wrong in general. It is wrong on that morning.
Caffeine is an alertness window, never a clock reset
Caffeine can hold you awake through a destination afternoon your body is reading as three in the morning. It does not move the underlying rhythm by a minute. Treat it as borrowed alertness, and time it against your bedtime rather than against how you feel. Gardiner 2023, a systematic review and meta-analysis of 24 studies, is where the cutoff here comes from: to avoid losing total sleep time, a 107 mg coffee should be taken at least 8.8 hours before bed, and a 217.5 mg serving of pre-workout at least 13.2 hours. Those are the two doses it modelled; anything between them is interpolation the review did not publish. Across the pooled studies caffeine cut total sleep time by about 45 minutes, so bad timing costs you the one night you can least afford.
On melatonin
Melatonin has been studied for jet lag and there is published evidence behind it. This page does not tell you whether to take it, at what dose, or at what hour, because the dose and the hour are the whole question, and a mistimed dose can move the clock the wrong way exactly as mistimed light can. That is a conversation with a clinician or a pharmacist, not something to settle with a calculator.
What this cannot tell you
It cannot tell you where your body clock minimum actually is. Two hours before habitual wake is a population average, and chronotype, recent sleep and recent light all move it. It does not model the flight: a red-eye, or a badly slept night before departure, changes the first day and appears nowhere in the arithmetic. It does not know your age, your medication or your shift pattern. It diagnoses nothing.
None of this is medical advice and none of it is a diagnosis. If your sleep has been disturbed for more than two weeks, well past the point where a trip should have worn off, it is worth raising with a clinician rather than a calculator.