You spend a third of your life asleep. You think you know what sleep is. You probably don't.

For most of human history, sleep was understood as simple rest — the body shutting down until morning. What sleep science has discovered since the landmark 1953 discovery of REM sleep is something far more remarkable: sleep is active, complex, and essential biological work. The brain during sleep is not offline. It is consolidating memories, processing emotions, clearing metabolic waste, and regulating systems throughout the body. And most people in the modern world are chronically short of it — without realizing how much that shortfall is costing them.

Sleep covers what sleep actually is and why it took decades of science to begin understanding it, how the circadian clock inside every cell of your body governs far more than your bedtime, what your dreaming brain is actually doing during REM sleep and why it matters, why sleep-deprived people are the worst judges of their own impairment, how artificial light has disrupted a biological rhythm our species depended on for millions of years, why modern society has built institutions and schedules that are actively hostile to sleep, and what the research says — carefully and honestly — about sleep and long-term health.

Drawing primarily on the work of sleep researcher Matthew Walker while acknowledging where his conclusions are contested, Sleep presents the current state of sleep science in accessible, engaging prose that takes the subject seriously without overstating what we know.

Part of the I'm No Expert, But series: short, accurate, accessible books on topics that are more surprising than most people expect. Readable in under an hour. The kind of book that leaves you thinking: I'm glad I know that now.