Jet lag: why east is harder, and how light resets your body clock
Jet lag is your body clock still keeping time where you left. It catches up by itself, but slowly — and when you get daylight decides whether you help it or hold it back.
What it is
After crossing at least two time zones, the body's internal clock is out of step with the local day. The result can be poor sleep, sleepiness in the day, trouble concentrating, feeling unwell and an upset stomach. After flying east the usual problem is not being able to fall asleep at night; after flying west, it is waking too early.
Why east is harder
The CDC gives average rates of adjustment of about 1 hour a day after flying east and 1.5 hours a day after flying west. The body clock finds it easier to run late than to run early. So six hours east, from New York to Paris, takes about six days to absorb, while eight hours west, from London to Los Angeles, takes about six too (8 ÷ 1.5 = 5.3).
For a very large eastward jump the arithmetic can favour going the other way round the clock. Sixteen hours east, from Los Angeles to Singapore in winter, is also eight hours west — about six days at the westward rate instead of sixteen at the eastward one.
Start before you leave
The CDC suggests shifting your sleep an hour a day for two or three days before the trip: an hour earlier each day before flying east, an hour later before flying west. Going to bed at 22:00 and then 21:00 before flying to Paris means two of the six hours are done before the plane leaves, and the rest takes about four days.
Light is what moves the clock
The body clock has a low point in the early hours, roughly two to four hours before you would normally wake. Bright light in the morning after that low point moves the clock earlier; light in the evening, before it, moves the clock later. The low point is set by the clock you arrived with, not the local one, which is why the right hours change day by day.
Take that New York to Paris trip, sleeping 23:00 to 07:00. On arrival your body's low point is still around 04:00 New York time — 10:00 in Paris. Bright morning light before then would push your clock the wrong way. So on the first day the advice is to keep out of bright light until about 10:00 and get plenty after it. Each day the low point comes about an hour earlier, until morning light is simply good for you again.
Flying west is gentler: the low point lands in the local evening or night, so late-afternoon and evening daylight helps, and very early morning light is the thing to avoid.
Trips too short to adjust
For trips of under about two days, the CDC's advice is not to adapt at all: keep sleeping, eating and being active on home time. If you can, put meetings inside the part of the local day that falls in your home-time daytime.
The rest of the advice
- Naps of 20–30 minutes can help you stay alert; longer naps make the next night harder.
- Caffeine can help daytime alertness at your destination; stop about six hours before bed.
- Alcohol may help you fall asleep but breaks up sleep, and both alcohol and caffeine can dehydrate.
- Melatonin in low doses can help shift the body clock, but it is not regulated by the US Food and Drug Administration, so quality varies. Ask a doctor or pharmacist first.
Source
- CDC Yellow Book — Jet Lag Disorder
Supports: the rates of adjustment by direction, shifting sleep before travel, timed light, staying on home time for short trips, naps, caffeine, alcohol and melatonin.
Updated: 23 April 2025. Checked: 14 September 2026.
The worked times above come from a simple model — the low point three hours before usual waking, moving at the CDC's average rates. Real body clocks vary.