The short answer

Light is one of the main signals that aligns the body clock with day and night. Bright light at one part of the body-clock cycle can move timing earlier; at another part it can move timing later. The right jet lag window depends on direction, time zones crossed and usual sleep.

Local morning is not enough information

After a large time-zone change, the clock on the wall and the clock in your body can describe different parts of the day. Light at 8 am locally may reach one traveler during their estimated body morning and another during their estimated body evening.

That is why a generic instruction to seek morning light can be incomplete for large shifts. The plan needs an estimate of where the body clock starts and which direction it should move.

Light can move timing in either direction

The CDC Yellow Book explains that bright light after the circadian low point generally promotes an earlier shift, while evening light generally promotes a later shift. The exact useful timing depends on the route and the traveler’s usual sleep.

Badly timed exposure can move the clock in the unwanted direction. Avoiding bright light can therefore matter as much as seeking it.

Why east and west are not the whole answer

Direction matters, but so do the size of the shift, flight timing, arrival and daylight-saving changes. A route that crosses many time zones can sometimes be approached in more than one direction around the 24-hour clock.

A complete plan needs more than a label such as eastbound or westbound. It needs the flights and the local times where the light guidance must fit.

How Offset uses light timing

Offset starts with your usual sleep and destination. It estimates body time, then places periods to seek or avoid bright light around the trip. Those windows sit alongside sleep and caffeine rather than replacing them.

The estimate is not a measurement of your exact biological phase. Personal response, indoor light, weather and the cabin can all change what exposure you receive.

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