A sleep cycle usually lasts about ninety minutes from start to finish.

The longer version

From the first drowsy moments to the end of the dream-filled phase, one pass through the stages takes about an hour and a half in adults. The figure is an average, and both individual nights and individual people vary around it. Later cycles contain more of the dream-filled phase and less of the deep phase than the first.

Because the cycle repeats, waking at a boundary feels different from waking mid-cycle, which is one reason alarm timing gets discussed. Infants run shorter cycles, and the rhythm lengthens through childhood. Most mammals show cycles scaled to their own body size. The regularity of the cycle is among the more reliable findings in sleep research and holds across laboratories.

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.