slow-wave sleep

Slow-wave sleep is the deepest, heaviest stage of ordinary sleep — the part of the night when you are hardest to wake and feel groggiest if someone shakes you out of it. It belongs to a family of stages called non-REM sleep (the quiet, mostly dreamless part of sleep, as opposed to REM sleep, when most vivid dreaming happens). In a standard map of the night it is labelled stage N3, and it usually dominates the first few hours after you fall asleep.

It gets its name from what the brain's electrical activity looks like. If you record brain waves with electrodes on the scalp (a technique called EEG), the busy, fast ripples of waking flatten into slow, tall, rolling waves — called delta waves, with a rhythm of only about one to four cycles per second. Tall (high-amplitude) means the wave swings far up and down; slow means few swings per second. These big, lazy waves appear because huge numbers of neurons fall into step, switching on and off together in a slow chorus instead of chattering on their own.

This deep stage is not just rest — it appears to be when a lot of repair and housekeeping happens. The body releases growth hormone, the brain clears out waste, and freshly learned facts seem to get filed into long-term memory. That is one reason a night cut short on deep sleep can leave you foggy and forgetful, and why young children, who are building brains and bodies fast, spend an especially large share of the night in slow-wave sleep.