The body clock runs on a roughly 24-hour cycle that persists without daylight cues.

The longer version

An internal pacemaker in the brain keeps time on its own, and in isolation experiments people housed without clocks or daylight settled into rhythms close to, but not exactly, twenty-four hours. Volunteers living in caves reported a similar drift, which convinced researchers the clock is genuinely endogenous. The cycle therefore has to be nudged daily to stay aligned.

That nudging comes from light reaching specialised receptors in the eye, along with regular mealtimes and activity. Damage to the pacemaker region abolishes the rhythm entirely. The term circadian comes from Latin words meaning about a day. Comparable self-sustaining timing appears across animals, plants and some single-celled organisms.

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.