Jet Lag Adjustment Calculator
Estimate how many days your body needs to adjust after crossing time zones, using the rule that eastward travel is harder than westward.
How it works
Days ≈ zones × 1 (eastward) or zones × 2/3 (westward), rounded up
The body clock resets at roughly one time zone per day, and the widely used travel-medicine rule of thumb is that eastward travel — where you must fall asleep earlier than your body wants — takes about a full day per zone, while westward travel adjusts faster at around two-thirds of a day per zone. These are averages for full adjustment; you will feel functional well before the clock is fully reset. Morning light after eastward flights and evening light after westward flights both speed the shift.
Worked example
Flying east across 6 time zones (say New York to Rome) takes about 6 × 1 = 6 days to fully adjust. The same 6 zones flown west takes about 6 × 2/3 = 4 days.
Sources: Jet lag (Wikipedia)
Frequently asked questions
Why is eastward travel worse?
Flying east shortens your day, forcing you to sleep before your body is ready — and the human clock naturally runs slightly longer than 24 hours, so lengthening the day (flying west) is easier than shortening it.
Does this mean I am useless for a week?
No. Full adjustment takes days, but most people function reasonably within a day or two. The estimate is the time for your sleep, appetite and alertness to be fully back on local time.
How can I adjust faster?
Seek bright light at the right time — morning light after flying east, late-afternoon light after flying west — stay hydrated, and shift meal and sleep times toward the destination a day or two before you leave.
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This calculator is for educational purposes. Double-check important results before acting on them.