Jet lag

How does crossing time zones disrupt the body's sense of when to sleep?

The obvious explanation is that flying is tiring and you have lost sleep. But test it. Someone who arrives rested, sleeps a full night in a dark quiet hotel and wakes at a reasonable hour is still fogged at three in the afternoon and staring at the ceiling at three in the morning — for days. Sleep debt does not behave like that; a night's sleep pays it off. So jet lag is not about how much sleep but about when. What could care about when?

Something must be keeping time, since sleepiness arrives at roughly the same hour whether or not you look at a clock. And there is a physical timekeeper: a small cluster of cells in the hypothalamus, the suprachiasmatic nucleus, running a cycle close to twenty-four hours by itself — in most people, slightly over. Hold on to that. From that cycle the body schedules melatonin, the daily swing in core temperature, cortisol, digestion and alertness.

But if it runs on its own, how does it know the hour outside? By light — specifically by retinal cells reporting brightness directly to the clock, separate from those that build the image you see. Which is why people blind with no light perception often drift, free-running past the day.

Now ask what the flight does. It does not touch your clock; your clock has no idea it moved. It swaps the world around the clock for a different one. Land in Tokyo from London and your body still keeps London time, eight hours behind: local midnight is your late afternoon, local breakfast your small hours. Two clocks that used to agree have been set against each other, and every signal your body sends is correct for a place you are not.

Then why not simply reset? Because the clock moves only so far per day — roughly an hour, with wide variation between people and directions. Here the "slightly over" earns its keep: stretching a day is the direction your clock already leans, so flying west is easier than flying east and demanding an earlier bedtime.

One more layer, which explains why the advice is not simply "get sunlight". What light does depends on when it arrives relative to your own cycle: light landing after your core temperature reaches its daily low, late in your biological night, pulls the clock earlier, while light before that low pushes it later. So on your first day, with your temperature minimum in the local early afternoon, a bright Tokyo morning arrives before your low and shoves you the wrong way.

Two things get run together. Plain travel fatigue — dehydration, cramped hours, a broken night — is real, is not jet lag, and resolves after one good sleep; jet lag is the misalignment, and persists through good sleeps. And the household detail is not decoration: the hypothalamic clock is master, but liver, gut and other tissues carry their own clocks, re-entraining at their own pace and nudged by meal timing as well as light. While those clocks disagree you get the diffuse malaise and digestive upset that sleepiness would not explain.

The honest caveat: the direction and rough rate above are well established, but "an hour a day" is a rule of thumb, individual variation is large, and the protocols built on the light-timing rule — and on timed melatonin — are worth following without being precise enough to promise a recovery date.

• Shift work, which produces the same misalignment without any travel and never fully resolves.
• Whether pre-adapting — shifting bedtime a little each night before departure — is worth the trouble.

Every term the collection defines is gathered in the glossary.