How minlag decides

Every step minlag gives you comes from a rule. minlag's engine sets every time on your iPhone; AI never does. This page explains each rule: what the app asks you to do, why, and where the idea comes from. The green boxes summarise a research finding in our own words, with a link to the original paper so you can read the authors' exact wording. The amber boxes are minlag's own choices: numbers we picked inside the ranges the research gives.

minlag offers general guidance, not medical advice. The plan as a whole has not been tested in a study. Check with your doctor before taking melatonin, or if you have a sleep condition.

1. Your body clock and its low point

What minlag does: it estimates the time your body is at its coldest, deepest-sleep point each night, and plans every light, sleep and caffeine step around it. It starts with your usual wake-up time and refines it with your real sleep from Apple Health, if you allow it. The dotted band on the timeline is your body's night.

Research finding, in our wordsIn people on a regular schedule, the body's temperature low usually falls a few hours before natural wake-up, and it is the anchor point for how light shifts the clock.

Eastman & Burgess 2009 · Burgess 2011 (see references).

minlag's choiceWe place the low at your usual wake-up minus 3 hours, then move it as your body adjusts.

2. Get bright light, keep light low

What minlag does: "Get bright light" turns the screen bright: go outdoors if you can. "Keep light low · sunglasses" turns it a dim blue-grey: sunglasses outdoors, dim rooms. In the dark it suggests a bright lamp or a light box. On trips that shift earlier, amber glasses are an optional tool in the evening. With real weather on, rain or cold in an outdoor light window adds a plan B ahead of time: a big window, a bright café, a lamp.

Research finding, in our wordsBright light shortly after the body's temperature low moves the clock earlier; bright light in the hours before it moves the clock later. The same light at the wrong time pushes you the wrong way.

Khalsa et al. 2003 · St Hilaire et al. 2012.

minlag's choiceLight windows are 4 hours long, trimmed to your waking hours, and dropped if shorter than 30 minutes.

3. Earlier or later, and "the long way round"

What minlag does: flying east usually means shifting your clock earlier; flying west, later. For big eastward jumps minlag may shift you later instead (Seattle to Tokyo is 16 hours ahead, but 8 hours later is the shorter way round).

Research finding, in our wordsMost people's clocks drift later more easily than earlier, so for large eastward shifts, delaying the long way round can be the easier path.

Eastman & Burgess 2009 · Huang et al. 2026.

minlag's choiceWe go the long way round for eastward shifts of 9 hours or more.

4. How fast you adjust

What minlag does: your body's night (the dotted band) moves toward the local night a little each day. The plan and the "on Tokyo time by" date follow from that pace, and from what you actually did.

Research finding, in our wordsLeft alone, the body clock adjusts roughly an hour a day, a bit faster when shifting later than earlier; well-timed light can speed this up.

Eastman & Burgess 2009 · Waterhouse et al. 2007.

minlag's choice1.5 h/day earlier and 2 h/day later when you follow the light steps; 1 and 1.5 h/day when you skip them. A step replaced with a substitute counts 70 %.

5. Shifting before you fly

What minlag does: for up to 3 days before departure, your bedtime and wake-up move about an hour a day toward your destination, with matching light steps, but never so far that your departure morning becomes impossible. When an early flight leaves no room to move your sleep, the prep days use light only.

Research finding, in our wordsShifting sleep and light gradually in the days before an eastward flight moved people's clocks part of the way before they left.

Burgess et al. 2003 · Revell & Eastman 2005.

minlag's choice1 hour a day, at most 3 days; the departure-day wake-up leaves time to reach the airport (3 h before international flights, 2 h domestic, plus an hour to get ready). Light-only prep days move bedtime at most 30 minutes.

6. On the plane

What minlag does: from boarding, the plan runs on destination time. It tells you when to sleep, nap or stay awake on board, depending on your cabin and whether you can sleep on planes. From the day before you fly, a small destination clock sits next to yours; from boarding it is the only clock.

General guidanceSleeping on the plane during the destination's night, and staying awake during its day, starts the switch before you land. Dry cabin air makes drinking water worthwhile.

CDC Yellow Book 2026, Jet Lag Disorder · Waterhouse et al. 2007.

minlag's choiceNo sleep in the first hour (take-off, meal service) or the last 45 minutes; a 90-minute nap mid-flight on long daytime flights; economy sleep split into blocks of up to 3 hours.

7. Caffeine

What minlag does: "Coffee OK" in the morning and a "Last caffeine" step well before bed. "Coffee now" moments help you stay alert after waking on board and through the arrival-day slump, never after the cut-off. Caffeine is never used to move your clock.

Research finding, in our wordsCaffeine taken even six hours before bedtime measurably disturbed sleep; evening caffeine can also nudge the body clock later.

Drake et al. 2013 · Burke et al. 2015.

minlag's choiceLast caffeine 6 hours before bed (5 or 8 if you set low or high sensitivity).

8. Naps and sleep debt

What minlag does: short naps only (30 minutes, before mid-afternoon) after you land; if you probably slept little on the plane, it adds a nap and an earlier bedtime.

General guidanceLong or late naps make it harder to fall asleep at the new bedtime.

CDC Yellow Book 2026.

minlag's choiceNap ≤ 30 min before 15:00; a sleep debt of 3 h or more adds a nap and moves bed up to 60 min earlier (not before 20:30).

9. Melatonin (optional, off by default)

What minlag does: if you turn it on, a 0.5 mg reminder in the early evening when you're shifting earlier, and only as a sleep aid on the first nights when shifting later. On trips that shift earlier it can also come before sleep on board. Always "check with your doctor".

Research finding, in our wordsA small dose (0.5 mg) taken in the afternoon or evening shifted the clock earlier about as well as a larger dose, when timed well.

Burgess et al. 2010 · CDC Yellow Book 2026.

minlag's choiceOn board: 30 minutes before sleep, on trips that shift earlier only; never two doses within 8 hours.

10. Short trips

What minlag does: for stays under 3 days it may keep you on home time — but only when your home sleep hours still fall mostly in the destination's night. Otherwise it adapts normally and offers home time as a choice.

General guidanceFor very short trips, staying on home time can avoid shifting twice.

CDC Yellow Book 2026 · Waterhouse et al. 2007.

minlag's choiceUnder 72 hours, and at least 5 hours of your home sleep inside the destination's 22:00–07:00 (or a shift of 3 hours or less).

11. Screens, water and exercise

What minlag does: "Dim lights · screens down" in keep-light-low windows and the hour before bed; water advice in the air and on arrival. Meals and exercise have their own section below.

Research finding, in our wordsReading on a bright tablet before bed delayed sleepiness and the body clock compared with a printed book.

Chang et al. 2015 (PNAS). Water and exercise are general comfort advice, not a way to adjust faster.

12. Meals, the kitchen and exercise

What minlag does: meals are on by default (turn them off in Settings › My plan). Breakfast comes about 30 minutes after the plan's wake-up, dinner ends about 3 hours before lights out, and lunch sits halfway between. Right after dinner, "Kitchen closes": nothing more to eat that night, so you go to bed without a full stomach. On the days before you fly, the meals move with your sleep toward the new time. On the plane you eat on destination time: have a meal served at a destination mealtime, skip one served at another hour (water is fine), and sleep through the ones served while you sleep. If you're hungry, "Tell minlag › I'm hungry now" answers from your plan.

Exercise (if you choose it): 30 minutes inside a light window, outside when it's light, never in the 3 hours before lights out, and not on travel days.

A light pre-flight fast (optional, off by default): eat lightly the day before you fly, little or nothing on board until breakfast time at your destination, then a full breakfast. Check with your doctor first. Not for pregnancy, diabetes or other medical conditions.

Research finding, in our wordsIn a lab study, a later meal schedule moved some of the body's clocks outside the brain, but not the main clock's timing marker. In animals, the timing of food can set a separate food clock.

Wehrens et al. 2017 · Fuller, Lu & Saper 2008. The fast is based on the Argonne diet, which troops used in one observational report (Reynolds & Montgomery 2002). Exercise timing: Youngstedt et al. 2019. These are reasons for the timing, not proof that eating or exercising this way helps you adjust.

minlag's choiceBreakfast 30 min after waking, dinner ending 3 h before lights out, lunch halfway; in the air, the usual meal service about 1 hour after take-off and 1 hour 30 before landing on long flights.

13. What your iPhone and Watch measure

Sleep (Apple Watch): when you really slept moves minlag's estimate of your body clock.
Time in Daylight (Apple Watch): credits a light window you didn't tap, and notices light at the wrong time.
Sleeping wrist temperature (Apple Watch): Health gives one value per night; a rise after travel is treated as a sign of a rough night, and minlag eases the pace. It is not a diagnosis.
Air pressure (iPhone and Watch barometer): a pressurised cabin sits at up to 8,000 ft (US regulation 14 CFR 25.841), far above most airports, so a pressure reading tells minlag you've taken off.
Location (iPhone, optional): one check when you open minlag around a flight: take-off over 250 km/h and 1,000 m above the airport (never speed alone, so a fast train doesn't count), real progress, landing. It never leaves your iPhone.
Time zone: when your iPhone switches to the arrival time zone, minlag logs the landing (with Undo).
Camera light check: "Is this bright enough?" estimates the light from the camera's exposure. A rough guide only.

What leaves your iPhone, and when: see Privacy.

14. The screen colour and the sky

What minlag does: by default the whole screen is one calm colour for what the plan wants now: night for sleep, dusk through screens down, a soft dawn after waking, bright for bright light. Press and hold the background to switch to Real sky: the sky where you are, then at your destination from boarding, drawn from the sun or from photos, with the real weather.

Sunrise and sunset: NOAA solar calculator equations. Weather: Apple Weather.

minlag's choiceDusk deepens to night across the screens-down window; dawn takes about 20 minutes; other changes ease over 5 minutes. The real sky can disagree with the plan (night in Tokyo while the plan says stay awake). The card always says what to do.

15. minlag's own numbers

Every number in an amber box is a minlag choice, not a figure from a single paper. Think one should change? Tell us at Support.

References

Burgess HJ. Using bright light and melatonin to reduce jet lag. In: Behavioral Treatments for Sleep Disorders, Elsevier, 2011. PDF
Eastman CI, Burgess HJ. How to travel the world without jet lag. Sleep Med Clin 2009;4:241–255. PubMed
Burgess HJ, Crowley SJ, Gazda CJ, Fogg LF, Eastman CI. Preflight adjustment to eastward travel. J Biol Rhythms 2003;18:318. Journal
Revell VL, Eastman CI. How to trick mother nature into letting you fly around or stay up all night. J Biol Rhythms 2005;20:353. DOI
Burgess HJ, Revell VL, Molina TA, Eastman CI. Human phase response curves to three days of daily melatonin: 0.5 mg versus 3.0 mg. J Clin Endocrinol Metab 2010;95:3325. PubMed
Khalsa SBS, Jewett ME, Cajochen C, Czeisler CA. A phase response curve to single bright light pulses in human subjects. J Physiol 2003;549:945. Journal
St Hilaire MA et al. Human phase response curve to a 1 h pulse of bright white light. J Physiol 2012;590:3035. Journal
Huang S, Bin YS, Caillaud C, Postnova S. Modeling the effects of flight itinerary on jetlag duration. SLEEP 2026. Journal
Waterhouse J, Reilly T, Atkinson G, Edwards B. Jet lag: trends and coping strategies. Lancet 2007;369:1117. PDF
Riedy SM, Williams SG. Jet Lag Disorder. CDC Yellow Book 2026. CDC
Drake C, Roehrs T, Shambroom J, Roth T. Caffeine effects on sleep taken 0, 3, or 6 hours before going to bed. J Clin Sleep Med 2013;9:1195. PubMed
Burke TM et al. Effects of caffeine on the human circadian clock in vivo and in vitro. Sci Transl Med 2015;7:305ra146. PubMed
Chang AM, Aeschbach D, Duffy JF, Czeisler CA. Evening use of light-emitting eReaders negatively affects sleep, circadian timing, and next-morning alertness. PNAS 2015;112:1232. DOI
US Code of Federal Regulations, 14 CFR 25.841, Pressurized cabins. eCFR
Wehrens SMT, Christou S, Isherwood C, et al. Meal timing regulates the human circadian system. Curr Biol 2017;27:1768. DOI
Fuller PM, Lu J, Saper CB. Differential rescue of light- and food-entrainable circadian rhythms. Science 2008;320:1074. DOI
Reynolds NC Jr, Montgomery R. Using the Argonne diet in jet lag prevention: deployment of troops across nine time zones. Mil Med 2002;167:451. PubMed
Youngstedt SD, Elliott JA, Kripke DF. Human circadian phase–response curves for exercise. J Physiol 2019;597:2253. DOI
NOAA Global Monitoring Laboratory. Solar calculation details. NOAA