Jet Lag Calculator

Jet lag happens when your internal clock conflicts with local time after crossing time zones. Recovery often takes about one day per zone crossed, though individual results vary widely with age, sleep habits, and direction of travel.

Eastward flights (losing time) tend to feel harder than westward ones for many travelers. Light exposure, meal timing, and short naps can help — but this calculator only estimates a rough day count, not a treatment plan.

Example: crossing 8 zones east on a 7-day trip might need roughly 8–12 days to feel fully adjusted, depending on the formula used. Shorter trips may mean you adjust just as it is time to fly home.

The direction asymmetry has a physiological basis. The human circadian clock free-runs slightly longer than 24 hours, so it drifts later more easily than earlier. Flying west asks the body to stay up longer, which it does willingly; flying east asks it to fall asleep early, which it resists — hence the rough guidance of about one day per zone eastward and rather faster westward.

Light is the strongest signal you can control, and timing beats quantity. Morning light after an eastward flight pulls your clock earlier, while evening light after a westward flight pushes it later. Getting the timing backwards can delay adjustment rather than speed it, which is why blindly seeking sun on arrival sometimes makes things worse.

Crossing the international date line changes the calendar but not the adjustment. What matters is the number of time zones between departure and arrival, not the date on your boarding pass, so a flight that appears to arrive the day before it left still shifts your clock by the zones actually crossed.

Long north-south flights cause travel fatigue without jet lag. A 12-hour flight within one or two zones leaves you exhausted, dehydrated, and stiff, but your body clock is intact — that resolves with one good night's sleep, whereas true jet lag persists for days regardless of how well you sleep.

For trips shorter than about three days, many travellers do better staying on home time: schedule meetings in your own waking hours if you can, rather than half-adapting and then reversing. The CDC's Travelers' Health pages cover light timing and medication questions; this tool estimates a day count only, not a treatment plan.

Eastward flights (losing time) typically feel harder than westward ones of equal length — hydration, morning outdoor light, and avoiding heavy meals on arrival often help more than the day-count estimate alone.

Short trips crossing fewer than three zones may not need full recovery days — adjust expectations for weekend getaways.