Dolphins rest one half of their brain at a time so they can keep swimming and surfacing.

The longer version

Unihemispheric slow-wave sleep lets one cerebral hemisphere power down while the other stays active enough to control swimming, keep the blowhole clear and watch for danger. The resting half shows the slow, high-amplitude waves typical of deep sleep, while the alert half keeps producing faster activity. The halves alternate over a rest period.

The arrangement solves a hard problem: dolphins breathe voluntarily and would drown if fully unconscious. Other marine mammals and some birds use the same trick, and birds can sleep with one eye open, keeping the opposite hemisphere alert. Eye closure follows the side that is asleep, so the open eye looks outward on the alert side.

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.