Jet lag: what crossing zones does to your clock
Your body keeps its own approximately-24-hour clock, and it does not move as fast as an aeroplane. The mismatch is the whole condition.
The short answer
Jet lag is the mismatch between your internal circadian clock and local time after crossing time zones faster than your body can adjust.
It shifts at roughly one hour a day travelling west and slightly less going east — which is why an eight-hour eastward flight can take the better part of a week to shake off.
What is actually happening
Your body runs a circadian rhythm on a cycle a little longer than 24 hours, governed by a cluster of cells in the hypothalamus and synchronised each day mainly by light.
That rhythm controls more than sleepiness: core body temperature, cortisol, melatonin, digestion and alertness all follow it. When you land somewhere eight hours away, every one of those is still running on the old schedule, and they do not all re-sync at the same rate — which is why jet lag feels like more than tiredness.
Why east is worse than west
Because the internal cycle is slightly longer than 24 hours, lengthening your day is easier than shortening it.
Travelling west you stay up later, which is the direction your body already drifts. Adjustment runs at roughly an hour a day, sometimes better.
Travelling east you must go to sleep and wake earlier than your body wants — working against the natural drift. Adjustment is slower, commonly quoted at around 30 to 60 minutes a day.
The practical result: London to New York is unpleasant for a day or two. New York to London is genuinely rough for three or four.
Rough recovery times
| Zones crossed | Westward | Eastward |
|---|---|---|
| 3 | 1–2 days | 2–3 days |
| 6 | 3–4 days | 4–6 days |
| 9 | 4–6 days | 6–9 days |
Individual variation is large. Age, sleep quality beforehand and how strictly you follow local time all move these figures considerably.
What is not jet lag
Travel fatigue — dehydration, cramped hours, disrupted sleep, airport stress — feels similar and is not the same thing. It resolves in a day and it happens on long north–south flights that cross no zones at all.
If you fly London to Cape Town, nearly the same longitude, you will arrive exhausted with no jet lag whatsoever. Anything that helps there is helping with fatigue, not circadian misalignment.
What genuinely helps
Light, correctly timed — the single strongest lever. Morning light after eastward travel, evening light after westward. Badly timed light actively delays adjustment.
Adopting local mealtimes and sleep times immediately, even when it feels wrong. Napping to "catch up" prolongs the mismatch.
Melatonin, which has reasonable evidence for eastward travel in particular, taken in the early evening at the destination. Availability and regulation vary by country.
More detail in how to avoid jet lag.
A note on what this site can tell you
Offsets, local times and sunrise/sunset at your destination are straightforward to look up, and they are what you need to plan light exposure and sleep timing.
Circadian rhythm itself is not something a clock app models. Anyone selling you that is overreaching.
Related: How to avoid jet lag · How long does jet lag last?
Timezone: World Clock Widget
A world clock, converter and live day/night map for iPhone — 325 cities, offline, no account.
Figures on this page are computed from the IANA time zone database. Last generated 2026-08-18.