# Jet lag planner: light and melatonin by the hour

The shift you have to cover is the destination's UTC offset minus your home offset. The hinge is the daily body temperature low, about three hours before your habitual wake time: light in the four hours after it moves the clock earlier, light in the four hours before it moves the clock later. The body clock covers roughly 1 h a day going east and roughly 1.5 h a day going west, so six zones east take about six days and six zones west about four. Three days of shifting sleep before departure take about two days off that.

## How this planner works

Four inputs, one row per day. Here is what happens in between.

- You pick your home time zone and your destination. The tool reads both UTC offsets for the day you travel, daylight saving included, and the difference between them is the shift you have to cover.
- You enter departure, your usual bedtime and wake time, and how many days you stay. Arrival is optional and only sharpens which calendar day the plan starts on.
- The plan puts the body temperature low at your wake time minus 3 h and moves it 1 h a day while the clock has to run earlier, 1.5 h a day while it has to run later. Around that hinge sit the bright-light and dark windows.
- The light and dark windows go into your calendar like appointments, together with target bedtime, wake time, the caffeine cut-off and the nap window. The arithmetic behind every number sits right under the plan.

## How fast the body clock actually moves

Left to itself the human clock runs at about 24 h 11 min, the figure Czeisler and colleagues measured under forced desynchrony in 1999. That small surplus is the reason the day stretches more easily than it shrinks. Aschoff's flight studies from 1975 put the difference in numbers: after westward flights the clock re-entrained at roughly 92 min a day, after eastward flights at roughly 57 min a day. This planner uses the rounded rule of thumb from that literature, about 1.5 h a day going west and about 1 h a day going east. Six zones east therefore take around six days, six zones west around four. A worked example: Munich to New York is six zones west, so at about 1.5 h a day the clock needs roughly four days. With a usual wake time of 07:00 everything hangs off a temperature low around 04:00, and three days of pre-shift before departure take about two days off the total. The calculator above runs the same arithmetic for your own flight, with the clock times filled in.

## The temperature low is the hinge

Light does not push the clock in one fixed direction. What decides is when it arrives, relative to the daily low point of core body temperature, which falls roughly three hours before habitual wake time. Khalsa and colleagues mapped that phase response curve in 2003: light in the hours after the low pulls the clock earlier, light in the hours before it pushes the clock later. St Hilaire and colleagues showed in 2012 that even a single one-hour pulse moves it measurably. The planner marks four hours on the side that pulls the clock the way this route needs as the bright-light window and three hours on the other side as the dark window, then walks both a little further each day.

## Three days of pre-shift before departure

Burgess and colleagues moved sleep one hour earlier a day for three days in 2003 and measured what the clock did with it: 0.6 h earlier under dim morning light, 1.5 h under intermittent bright light, 2.1 h under continuous bright light in the morning. Revell and colleagues added 0.5 mg of melatonin in the afternoon in 2006 and reached about 2.5 h. Two hours of head start, at this planner's 1 h a day, is roughly two days of the journey done before boarding, but only with the morning light; the sleep shift on its own bought less than a third of that. That is why the pre-shift checkbox takes about two days off the plan and puts a light window on each of the three home days. Going west the same checkbox moves sleep later instead, at the upper end of the 30 to 60 min a day Roach and Sargent suggest for the days before a westward flight; there the sleep move itself is the intervention, because the light windows of those nights fall into sleep.

## What the melatonin trials found

The Cochrane review by Herxheimer and Petrie reviewed ten randomised trials in 2002. Nine of them found that melatonin taken close to the destination's bedtime reduced jet lag after crossing five or more time zones, with a number needed to treat of about two; the summary now on the Cochrane website counts eight of the ten. Those trials dosed at bedtime to blunt the symptoms; this planner uses the earlier, clock-shifting timing from Revell and Burgess instead, so that number needed to treat belongs to the bedtime protocol rather than to the time in your plan. Doses between 0.5 and 5 mg worked about equally well, except that people fell asleep faster and slept better on 5 mg than on 0.5 mg; what causes trouble is mistiming, because taken early in the day melatonin makes people sleepy and delays adaptation. The American Academy of Sleep Medicine rated melatonin for jet lag as Standard, its strongest recommendation grade, in the 2007 practice parameters by Morgenthaler and colleagues. Revell and colleagues gave 0.5 mg five hours before the usual bedtime in 2006 and found it as effective as 3 mg, which is where this planner's five hours come from; Roach and Sargent put the optimum for 0.5 mg at about five and a half hours before bedtime in 2019 and work with 3 mg at about six and a half, so the hour moves with the dose. Burgess and colleagues mapped melatonin's own phase response curve in 2010: like light, it works by when it is taken more than by how much, and that curve has a delaying arm in the morning which this planner does not use.

## Melatonin and German law

In Germany the status of melatonin is contested rather than settled. BfArM and the BgVV, the institute whose tasks later passed to the BfR, held in 1996 that melatonin is a medicinal product independently of dose, and BfArM went on to treat products at 0.5 to 5 mg a day as authorisation-requiring, a position on record in the Cologne administrative court's 2014 judgment. The BfR's own 2024 opinion no longer repeats the dose-independent line; it warns about the products as food instead. Products at up to 1 mg a day are sold as food supplements under the EU health claims, and courts have ruled both ways: the Cologne court called 0.5 mg capsules medicinal products in 2014, a higher administrative court did not in 2021. Since 23 May 2026 there is a third layer: medicines with 3 mg per dose unit, at most 3 mg a day and packs of up to 30 mg, taken by adults for jet lag, no longer need a prescription in Germany. The 0.5 mg in this plan sits at the bottom of the disputed range, and products at that dose are on drugstore and online shelves as supplements, so what is unsettled is their legal status rather than where to find them. Epilepsy or blood thinners make it a doctor's question first. Melatonin makes people sleepy, so driving after taking it is not part of any plan here.

## Why the caffeine cut-off sits eight hours before bed

Burke and colleagues gave people the caffeine of a double espresso three hours before bedtime in 2015 and measured a circadian delay of about 40 minutes, roughly half of what bright light did in the same experiment. Caffeine does not only keep you awake, it also nudges the clock later while an eastward plan is trying to pull it earlier. Drake and colleagues had already shown in 2013 that 400 mg six hours before bed costs more than an hour of sleep, without the drinkers noticing much. Eight hours is the margin that keeps both effects out of the plan. Caffeine at the other end of the day is a different lever: Beaumont and colleagues gave 300 mg of slow-release caffeine at 08:00 for five days after a seven-zone eastward flight in 2004 and measured less daytime sleepiness, with sleep quality paying part of the bill, and Pierard's group had found in 2001 that the same morning dose resynchronised the cortisol rhythm about as fast as 5 mg of melatonin. Neither is in this plan, which only keeps the evening clear.

## Eight zones east can be easier the long way round

From about eight time zones east, the clock sometimes stops trying to move earlier and drifts later instead, all the way around. Eastman and Burgess described this antidromic re-entrainment in 2009: instead of advancing eight hours the clock delays sixteen. That is more hours and more days, but it runs in the direction the clock drifts anyway, which is why the planner offers the delay route as a second option from eight zones east. On that route the windows swap sides: the bright light sits before the temperature low and the dark window after it. On the first days that puts both in the local daytime, from late morning into the afternoon; they only walk into the evening as the plan runs.

## How this planner compares with the other tools

Entrain comes out of a university lab: Serkh and Forger published the optimal-control mathematics behind it in 2014, which makes it worth checking a plan against. Timeshifter is the polished commercial app; its widely quoted 96.4 per cent is the share of travellers who followed its advice and then reported no severe or very severe jet lag, out of about 130,000 of the company's own post-trip surveys rather than a controlled trial, so it measures what compliant users said afterwards, not what their clocks did. This planner is bilingual, free, and prints every step of its arithmetic under the result, so you can see where each time comes from and disagree with it.

## Where this plan is only an estimate

The temperature low is estimated from your wake time, never measured, and even the population figure has a spread: Eastman and Burgess put it 3 to 4.5 h before wake time after an 8 h night, and the planner uses the round 3 h at the near edge of that. Very early or very late chronotypes sit further out again, and age moves it too. The rates of 1 h and 1.5 h a day come from lab studies and flight cohorts, where light, meal times and schedules were far more controlled than on a real trip, and they describe clocks left to sort themselves out; Eastman and Burgess plan with 1.5 h a day advancing and 2 h a day delaying when the light is timed well, so a plan you actually follow may land a day or two sooner than the row count says. Meal timing, exercise and alcohol move the clock too, weakly and less predictably than light, and none of them is modelled here; the article covers what is known about them. Daylight saving is resolved for the departure and arrival moments, so a plan that crosses a clock change carries a note. Everything here is a rule of thumb with a known spread, not a prediction for your particular clock.


## FAQ

### Is a trip across two time zones worth planning?

The literature says no. Waterhouse and colleagues put it plainly in 2007: crossing fewer than three time zones should not cause significant jet lag, because the clock drifts into place on its own within a day or two, and a schedule mostly costs attention. The tool says the same and skips the day rows at two zones or fewer, and on trips of two days or less.

### Why is sleeping more after landing not enough?

Sleep debt and jet lag are two different problems. Extra sleep repays the debt, but the body clock only moves when light reaches it at the right time relative to the temperature low. A long lie-in through the destination's morning can push the clock the wrong way while it feels like catching up.

### Where do the light windows sit on a westward flight?

Mirrored. Going west the clock has to run later, so the bright-light window sits in the four hours before the temperature low and the dark window in the three hours after it. On the first days the low still sits in the local afternoon or evening, so the bright window lands ahead of it and the dark window right behind it; both walk later each day until the dark window has moved into your sleep.

### Is eight zones east really easier the long way round?

Sometimes. Eastman and Burgess documented that above about eight zones the clock often delays sixteen hours rather than advancing eight, because later is the direction it drifts anyway. Sixteen hours at 1.5 h a day is more days than eight hours at 1 h a day, so the tool shows both counts and leaves the choice with you.

### Is melatonin legal in Germany?

That is genuinely disputed. The federal institutes classify products at 0.5 to 5 mg a day as medicinal products and call that view dose-independent, while products at up to 1 mg a day are sold as food supplements under the EU health claims, and courts have ruled both ways. The 0.5 mg in this plan sits inside that disputed range, so a pharmacy is the place to ask. Epilepsy or blood thinners make it a doctor's question first, and driving after intake is not part of any plan here.

### How accurate is wake time minus three hours?

It is a population average, and the average itself has a spread: Eastman and Burgess put the temperature low 3 to 4.5 h before wake time after an 8 h night, and the planner uses the round 3 h at the near edge of that, the figure the light-timing literature works with. Extreme chronotypes and older adults sit further out again, and every window in the plan moves by the same amount.

### What about the flight home?

The same arithmetic runs in reverse. Swapping home and destination and entering the return date gives the plan for the way back. The direction usually flips as well, so the daily rate changes from 1 h to 1.5 h or the other way round.

## Sources

- Czeisler CA, Duffy JF, Shanahan TL, et al.. (1999). Stability, precision, and near-24-hour period of the human circadian pacemaker. Science. https://doi.org/10.1126/science.284.5423.2177
- Eastman CI, Burgess HJ. (2009). How to travel the world without jet lag. Sleep Medicine Clinics. https://doi.org/10.1016/j.jsmc.2009.02.006
- Khalsa SBS, Jewett ME, Cajochen C, Czeisler CA. (2003). A phase response curve to single bright light pulses in human subjects. The Journal of Physiology. https://doi.org/10.1113/jphysiol.2003.040477
- St Hilaire MA, Gooley JJ, Khalsa SBS, et al.. (2012). Human phase response curve to a 1 h pulse of bright white light. The Journal of Physiology. https://doi.org/10.1113/jphysiol.2012.227892
- Burgess HJ, Crowley SJ, Gazda CJ, et al.. (2003). Preflight adjustment to eastward travel: 3 days of advancing sleep with and without morning bright light. Journal of Biological Rhythms. https://doi.org/10.1177/0748730403253585
- Revell VL, Burgess HJ, Gazda CJ, et al.. (2006). Advancing human circadian rhythms with afternoon melatonin and morning intermittent bright light. The Journal of Clinical Endocrinology and Metabolism. https://doi.org/10.1210/jc.2005-1009
- Herxheimer A, Petrie KJ. (2002). Melatonin for the prevention and treatment of jet lag. Cochrane Database of Systematic Reviews. https://doi.org/10.1002/14651858.CD001520
- Morgenthaler TI, Lee-Chiong T, Alessi C, et al.. (2007). Practice parameters for the clinical evaluation and treatment of circadian rhythm sleep disorders. Sleep. https://doi.org/10.1093/sleep/30.11.1445
- Roach GD, Sargent C. (2019). Interventions to minimize jet lag after westward and eastward flight. Frontiers in Physiology. https://doi.org/10.3389/fphys.2019.00927
- Burgess HJ, Revell VL, Molina TA, Eastman CI. (2010). Human phase response curves to three days of daily melatonin: 0.5 mg versus 3.0 mg. The Journal of Clinical Endocrinology and Metabolism. https://doi.org/10.1210/jc.2009-2590
- Burke TM, Markwald RR, McHill AW, et al.. (2015). Effects of caffeine on the human circadian clock in vivo and in vitro. Science Translational Medicine. https://doi.org/10.1126/scitranslmed.aac5125
- Drake C, Roehrs T, Shambroom J, Roth T. (2013). Caffeine effects on sleep taken 0, 3, or 6 hours before going to bed. Journal of Clinical Sleep Medicine. https://doi.org/10.5664/jcsm.3170
- Serkh K, Forger DB. (2014). Optimal schedules of light exposure for rapidly correcting circadian misalignment. PLoS Computational Biology. https://doi.org/10.1371/journal.pcbi.1003523
- Waterhouse J, Reilly T, Atkinson G, Edwards B. (2007). Jet lag: trends and coping strategies. The Lancet. https://doi.org/10.1016/S0140-6736(07)60529-7
- Beaumont M, Batejat D, Pierard C, et al.. (2004). Caffeine or melatonin effects on sleep and sleepiness after rapid eastward transmeridian travel. Journal of Applied Physiology. https://doi.org/10.1152/japplphysiol.00940.2002
- Pierard C, Beaumont M, Enslen M, et al.. (2001). Resynchronization of hormonal rhythms after an eastbound flight in humans: effects of slow-release caffeine and melatonin. European Journal of Applied Physiology. https://doi.org/10.1007/s004210100418

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