Light is the strongest signal your body clock gets, so it's your best tool against jet lag, and also the easiest to get wrong. The rule: light in the hours before the low point of your body's night pushes the clock later; light in the hours after it pulls the clock earlier. Get light on the right side, and you adjust faster. Get it on the wrong side, and you can make jet lag worse.
How light moves the body clock
Your body clock, in a small region of the brain called the suprachiasmatic nucleus, takes its cue from light reaching special cells in the eye. The effect depends on timing. Sleep researchers describe it with a phase response curve: light before your core body temperature reaches its lowest point (usually a couple of hours before your normal wake-up time) delays the clock; light after that point advances it.
- Evening light (before the low point) → your clock runs later. Useful after flying west.
- Morning light (after it) → your clock runs earlier. Useful after flying east.
The catch: your internal morning isn't local morning
Right after a long flight, your body is still on home time. After flying east across many time zones, local early morning can fall during your body's night, before its temperature low. Bright light then would push your clock the wrong way. That's why advice for big eastward trips often says to avoid early morning light for the first day or two, and seek late-morning light instead, moving the window earlier each day.
A practical guide
| Trip | Seek light | Avoid light |
|---|---|---|
| East, up to about 8 hours | Morning, from about 8 AM | Very early morning, and late evening |
| East, 8 hours or more | Late morning and early afternoon, moving earlier each day | Early morning, for the first 2 days |
| West, up to about 8 hours | Late afternoon and evening | Early morning, before about 8 AM |
| West, 8 hours or more | Midday and afternoon | Late evening, for the first 2 days |
A general guide, after the timings Jim Waterhouse and colleagues set out in The Lancet (2007). The jet lag calculator tells you which way your clock needs to move for any trip, and taking the shorter way round means some long eastward trips are really westward for your body.
How to get light, and avoid it
- Get outside. Daylight, even on a cloudy day, is many times brighter than indoor lighting. A walk is better than a window.
- Check when the sun is up at your destination: the sunrise and sunset calculator covers any city with an airport. In winter at high latitudes, the window may be short.
- To avoid light, wear sunglasses outside and stay in dim rooms. Turn screens down in the evening.
- Light boxes help some people, especially when daylight is scarce, but follow the instructions on timing.
Light plus the rest of the plan
Light works best with everything else: shifting your sleep before you go, eating at local times, and keeping a regular wake-up time. Our day-by-day jet lag plan puts it all together.
Rehearse the trip
In Flight, a long-haul flight becomes a focus session at your desk, as long as the real trip. It won't shift your body clock, but it's a good way to get some work done before you go.