## TL;DR

Sleep occupies one-third of human life and is essential for health. Functions: memory consolidation, metabolic waste clearance (glymphatic system), immune function, emotional regulation. Adults need 7-9 hours. Sleep deprivation impairs cognition comparable to alcohol intoxication.

## Core Explanation

Stages: NREM (N1-N3, deep sleep for physical restoration) and REM (rapid eye movement, dreaming, memory consolidation). Circadian rhythm: ~24.2 hour internal clock set by light (suprachiasmatic nucleus). Melatonin: hormone signaling darkness. Adenosine: builds during wakefulness → sleep pressure. Caffeine blocks adenosine receptors. Chronic sleep deprivation linked to: Alzheimer's, cardiovascular disease, obesity, depression.

## Further Reading

- [Why We Sleep (Matthew Walker)](https://www.simonandschuster.com/books/Why-We-Sleep/Matthew-Walker/9781501144325)