Why We Get Jet Lag

Inside your body there is a kind of clock. It tells you when to feel awake and when to feel sleepy. It also controls things like body temperature, hunger and hormones. This clock follows a cycle of about 24 hours. Light is the main thing that sets it.

When you fly a long way east or west, you cross time zones very quickly. Your body clock stays on the time of the place you left. But the new place has a different time. Your body wants to sleep in the afternoon, or feels wide awake at 3 a.m.

This is jet lag. You feel tired, you cannot sleep well, you are hungry at strange times, and it is hard to think clearly. For many people, flying east feels worse than flying west.

Your body clock slowly changes to the new time. It usually takes about one day for each time zone you crossed. Going outside in daylight helps it change faster.

Nearly every cell in the body runs on a roughly 24-hour rhythm, coordinated by a master clock in the brain. This circadian system governs not just sleepiness and alertness but body temperature, digestion, hormone release and reaction time. Its main external cue is light: bright light in the morning nudges the clock earlier, light in the evening nudges it later.

Air travel creates a mismatch. Fly from London to Tokyo and you arrive nine hours ahead, but your internal clock is still running on London time. It expects darkness and sleep when Tokyo is having lunch. Realigning takes days, because the clock can only shift by an hour or two per day.

The symptoms — fatigue, fragmented sleep, poor concentration, appetite at odd hours, digestive upset — reflect the whole system being out of phase, not just tiredness.

Eastward travel is generally harder than westward. Flying west lengthens your day, and the clock finds it easier to delay than to advance; flying east compresses your day and forces the clock to jump forward, which it resists. Strategies that help all work by giving the clock clear signals: seeking daylight at the right times, timing meals to the destination, and, for some travellers, small doses of melatonin in the evening.

Jet lag is the visible symptom of an invisible institution: a network of biological clocks, present in almost every tissue and synchronised by a pacemaker in the hypothalamus, that anticipates the daily cycle rather than merely reacting to it. Because the system is predictive, it cannot be reset instantly. It is entrained — gradually pulled into alignment — chiefly by light detected through specialised cells in the retina, and it shifts at a limited rate of roughly one hour per day.

Rapid transmeridian travel exploits the one thing the clock cannot do: keep up. Cross six time zones and every downstream rhythm — core temperature, cortisol, melatonin, gut motility, cognitive throughput — is now running on origin time while the environment demands destination time. The malaise, the broken sleep, the mid-afternoon collapse and the 4 a.m. clarity are all manifestations of internal desynchronisation, and they persist until the slowest-adjusting rhythms have caught up, which can take longer than the sleep–wake cycle itself.

The east–west asymmetry has a clean explanation. The unforced period of the human clock runs slightly longer than 24 hours, so it delays more readily than it advances. Westward travel asks it to delay, extending the day; eastward travel asks it to advance, and it resists. The rule of thumb — about a day of recovery per time zone eastward, somewhat faster westward — follows directly.

Countermeasures are, in essence, attempts to supply the clock with unambiguous cues at the right phase: strategically timed bright light and darkness, meals aligned to destination time, and in some protocols exogenous melatonin. None of it works quickly, because the underlying constraint — a biological system built for the slow rotation of the Earth, not the speed of a jet — is not negotiable.