Free Jet Lag Calculator
Enter your route, flight time and chronotype to get an instant day-by-day plan: when to seek light, when to avoid it, when to sleep, and when melatonin makes sense.
🛰️ Based on circadian sleep research (NASA, Oxford, Nature)
How jet lag adjustment works
Light is the strongest cue for your internal clock. At the right time of day, bright light shifts your rhythm in the direction you want — at the wrong time, it shifts it the wrong way. The calculator tells you, day by day, when to get outside into daylight and when to wear sunglasses or keep things dark instead.
Instead of switching abruptly once you land, the plan shifts your bedtime gradually — about an hour a day eastward, up to an hour and a half westward. Ideally start one to three days before your flight so your body arrives already partly adjusted.
Traveling west means staying awake longer — most people find this easier, since the internal clock naturally runs a bit longer than 24 hours. Traveling east means going to sleep earlier, which is harder. That's why the calculator plans more adjustment days for eastbound flights.
It's not the dose that matters, but the timing. For eastbound trips, a small amount in the early evening helps you feel sleepy earlier; for westbound trips, it's more about preventing nighttime waking. The calculator marks the right window for you — but please check with a doctor if you're unsure about taking it.
Flyly's plan is based on the phase-response curve of circadian chronobiology — the same model used in sleep research and space medicine for shift work and time zone changes:
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