The body clock, and why a late landing feels like afternoon

The body clock is the internal rhythm that decides when you feel sleepy, when you feel hungry and when you feel alert. It is set chiefly by daylight and by meal times, and it moves slowly, by roughly an hour a day. Cross six time zones in one flight and it stays behind, still running on the place you left.

Flying west into a late landing

A traveller arriving from Asia or the Gulf late at night often feels strangely awake, because the body believes it is mid-afternoon. Sleep may not come until three or four. For that traveller, a longer session, two hours rather than one, fills the waking stretch and brings sleep closer. The guide on choosing a length after a late landing goes through the options.

Flying east on a red-eye

A traveller arriving from North America on an overnight flight lands with a body that believes it is the middle of the night. They are sleepy in the morning and wide awake in the evening. A short session in the morning, a nap of no more than ninety minutes and plenty of daylight afterwards usually help the clock move.

What resets it

Daylight in the morning is the clearest signal the clock receives in an ordinary day, and meals at local times come next. Darkness in the evening, with screens put away, helps too. A session in the evening, followed by a dark room and sleep, fits into that pattern well.

What it does not decide

The body clock shapes when a session helps and how long it should be. It does not shape what the session includes; that is for your therapist to decide, and she tells you her limits before it begins.