Jet lag appears when travel crosses time zones faster than the body clock can adjust.

The longer version

Flying across several meridians leaves the internal schedule set to the departure zone, so sleepiness arrives in the middle of the new afternoon while local night brings wakefulness. A traveller may feel wide awake at three in the local morning and desperate to sleep through the local afternoon. The mismatch is a timing problem, not a shortage of sleep.

Symptoms include disturbed appetite, sluggish digestion and poor concentration, because every organ carries its own daily schedule and each resyncs at a different pace. Aircrew and frequent travellers report the same lag, and it tends to grow with the number of zones crossed. The clock can only move a limited amount per day, so the gap closes slowly.

Where this entry sits

This collection gathers curious details about sleeping and dreaming, from nightly brain stages and body clocks to naps, yawns and unusual animal habits. It also covers how bedtime customs have changed across centuries, without offering any advice. The full list sits on the Sleep and Dreams page, where every entry is listed in order.