Sleep happens in cycles that repeat several times during a single night.

The longer version

Across one night the brain moves through a repeating sequence: light drowsiness, slow deep sleep, then a phase with rapid eye movement, before climbing back toward lighter stages. The whole round trip takes roughly an hour and a half in healthy adults, and a full night normally contains several such rounds.

The rounds are not identical copies. Early rounds hold most of the deep slow-wave portion, while later ones stretch the rapid-eye-movement phase, so the night's architecture shifts as it progresses. Most mammals studied show comparable ultradian patterns, though cycle length varies considerably with body size.

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.