Jet lag is your body clock lagging behind the local time. The number of zones you cross and the direction you travel determine how long it lingers — and knowing the estimate lets you plan around it instead of fighting it blindly.
Why direction matters
Crossing time zones eastward shortens your day, which is harder for a body clock that naturally runs a little long; westward lengthens it and feels gentler. That asymmetry is why a New York–Tokyo flight (eastward, 13 zones) hits harder than the return.
The day-per-zone guideline
As a rule of thumb, expect about one day of adjustment per one or two time zones. This tool turns that into a number you can plan around — booking the tricky meetings for later in the trip, for instance.
Resetting the clock
Light is the strongest cue. Morning exposure pulls your clock earlier; evening light pushes it later. Pair it with consistent local meal and sleep times, and most travellers resync within a few days.
Frequently asked questions
Roughly one day per time zone crossed, though eastward travel tends to take longer than westward. This calculator estimates your recovery in days.
Eastward is generally worse — you lose time, and your internal clock resets more slowly when moving earlier. Westward trips usually feel milder.
Get bright light at the right time (morning for west-to-east arrivals), hydrate, stay active, and follow local meal and sleep times from day one.