Deep sleep is the hardest stage to wake from, and it dominates the first part of the night.

The longer version

Slow-wave sleep produces large, synchronized waves across the cortex, and the threshold for waking rises steeply. A sleeper pulled out of it often feels disoriented for several minutes, answering slowly and misjudging the time. This stage concentrates in the first half of the night and shrinks with every later cycle.

That concentration reflects a pressure that builds during waking hours and discharges early in sleep, so the deepest waves arrive when the pressure is highest. By dawn it has largely eased, leaving lighter and more dream-filled stages. The gradual thinning explains why a noise that fails to wake a sleeper at midnight succeeds easily by morning. Children show far more of the stage than older adults do.

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.