Jet lag planner: light and melatonin by the hour

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 in this planner's arithmetic three days of pre-shift before departure take about two days off the total, an assumption on a westward route rather than a measured result. The calculator above runs the same arithmetic for your own flight, with the clock times filled in.

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.

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. The planner counts that westward head start as the same two days, a simplification no trial has tested.

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, set on the body clock, 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; this planner places that moment on the body clock, 10 h before the temperature low, which at home with 8 h of sleep is those five hours; 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 body's morning, which is why this planner never puts the dose there, even when the local clock already says evening. Melatonin shifts the clock in both directions, but the delaying arm of its curve is mapped less sharply, and Eastman and Burgess left it out of their travel plans because there is no experimental evidence that it adds anything to well-timed bright light, so this planner only places the melatonin moment on plans that move the clock earlier.

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 this plan's margin. A 2023 meta-analysis of 24 studies by Gardiner and colleagues puts the gap that protects total sleep at about 9 hours for one regular coffee (107 mg) and about 13 hours for a 217 mg serving, so bigger doses need more room. 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.

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.

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.

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.

The studies behind the numbers on this page. A few figures come from elsewhere and are named in the text where they appear: Aschoff's 1975 flight data, the CDC's daily rates and nap length, and the German dose limits.

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