Yawning is contagious in humans, and seeing someone else yawn often triggers your own.

The longer version

Seeing, hearing or even reading about a yawn can trigger one within seconds, and the effect appears in children from about age four onward, once certain social perception circuits mature. The trigger can also come from a photograph or a video, and the response is hard to suppress once it begins. It spreads more readily between people who know each other.

The response has been reported in chimpanzees, dogs and some other social mammals, suggesting an old mechanism for synchronising alertness within a group. Brain imaging points to regions involved in imitation and social awareness rather than a simple reflex. Contagious yawning is much reduced in some conditions affecting social processing, which has made it a useful research probe.

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.