The jet lag planner.
Jet lag is your body clock being out of step with local time. Tell us your trip and get an estimate of how long it may take to adjust - plus exactly when to seek and avoid light, the strongest lever you have.
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Method & sources
Estimates use the established rate at which the body clock re-entrains: about one time zone per day flying east (advancing the clock is harder) and up to ~1.5 per day flying west (delaying is easier), because the human clock naturally runs a little longer than 24 hours. Light advice follows the core-body-temperature-minimum rule - bright light after that minimum (~2 hours before your usual wake) advances the clock, while light before it delays the clock.
- Eastman CI, Burgess HJ. How to travel the world without jet lag. Sleep Med Clin. 2009;4(2):241–255.
- Sack RL. Jet lag. N Engl J Med. 2010;362(5):440–447.
- Roach GD, Sargent C. Interventions to minimize jet lag after westward and eastward flight. Front Physiol. 2019;10:927.
- CDC Yellow Book - Jet Lag.
- Sleep Foundation - Jet Lag.
Why east is harder than west.
Your internal clock naturally runs slightly longer than 24 hours, so it is easier to stay up later (a delay) than to fall asleep earlier (an advance). Flying west delays your clock - easier, and you re-sync up to about 1.5 time zones a day. Flying east forces an advance - harder, at roughly one zone a day. That is why most people find eastward trips rougher.
Light is the main lever
Light is the strongest signal for your body clock, and timing is everything. The key reference point is your core-body-temperature minimum - about two hours before your usual wake time. Bright light after that point pushes your clock earlier (an advance); bright light before it pushes your clock later (a delay). So flying east you seek morning light and avoid late-evening light; flying west you seek evening light and protect your eyes from early-morning light.
A head start helps
Shifting your sleep by 30 to 60 minutes a day for a few days before you fly - earlier for eastward trips, later for westward - gives your clock a running start. On the plane, set your watch to the destination and try to eat and sleep on the new schedule.
What this tool isn't
It's general travel guidance, not medical advice, and it does not prescribe any medication or supplement dose. For sleep aids, melatonin, or if you have a medical condition, talk to a pharmacist or doctor.