Why do we sleep?
Sleep clears brain waste, files memories, and resets the body, no single reason, but several jobs at once.
Sleep is not one thing
We tend to imagine sleep as a single off-switch, but the brain runs a structured program. Across a night you loop through distinct stages, lighter ones, the slow electrical waves of deep sleep, and the vivid, active state of REM, each doing different work.
The brain takes out the trash
During deep sleep the spaces between brain cells widen, and cerebrospinal fluid flushes through more freely. This glymphatic flow carries away metabolic byproducts that build up during waking hours, including amyloid-beta, a protein tied to Alzheimer's disease.
Memories get sorted overnight
The hippocampus replays the day's events while you sleep, transferring fragile traces into longer-term storage in the cortex. Skip sleep after learning something and the memory is measurably weaker the next day.
What goes wrong without it
There is still no single agreed-upon ‘reason’ for sleep, restoration, memory, and energy conservation all play a part. What is certain is that no animal has evolved away the need for it, which suggests its benefits are non-negotiable.