Light sleep, deep slow-wave sleep and REM sleep: what happens in the brain during each stage, how they cycle through the night, and how sleep changes across life.
Sleep & Consciousness
Beginner
The Stages of Sleep
The Editors
August 20, 2026
3 min read
In plain English
Sleep isn't one state. You cycle through light sleep, deep sleep and dreaming (REM) sleep about every 90 minutes, four to six times a night. Each stage has its own brain-wave pattern. Deep sleep is concentrated early in the night and REM sleep later toward morning.
Beginner level: No background needed. Everyday language, everyday examples — the place to start.
Concepts to know first
Go deeper next
Before the 1950s, sleep was widely thought of as a passive shutting down of the brain. The electroencephalogram (EEG) showed it is an active, structured process.
Discovery of REM sleep
In 1953, at the University of Chicago, Eugene Aserinsky and Nathaniel Kleitman noticed periods during sleep when the eyes darted rapidly under closed lids and brain waves resembled wakefulness. People woken from these periods usually reported vivid dreams. They had discovered rapid eye movement (REM) sleep. William Dement and Kleitman then described the regular cycling of sleep stages through the night.
The stages
Sleep is divided into non-REM (stages N1, N2, N3) and REM sleep.
N1 is the drowsy transition, with slowing brain waves and sometimes sudden muscle jerks (hypnic jerks). N2 is light but true sleep, making up roughly half the night in adults. Its EEG shows sleep spindles — short bursts of 11–16 Hz waves generated by the thalamus, linked to memory consolidation — and K-complexes, large single waves. N3, or slow-wave sleep, is deep sleep, dominated by large, slow delta waves. It's hard to wake someone from N3, and they may be groggy. Growth hormone is released, and sleepwalking and night terrors arise from this stage.
REM sleep has fast, low-voltage brain waves like wakefulness, rapid eye movements, irregular breathing and heart rate, and vivid dreaming. The body's muscles (except those for breathing and the eyes) are actively paralysed by brainstem circuits — REM atonia — which prevents us acting out our dreams.
Cycles across the night and life
A cycle through these stages takes about 90 minutes, repeating four to six times. Early cycles are rich in deep N3 sleep; later cycles have longer REM periods — why vivid dreams are common just before waking. Newborns spend about half their sleep in REM; deep slow-wave sleep declines substantially with age, and older adults wake more often during the night.
Everyday examples & things to try
Try it
Time your alarm to a cycle
Some people find waking at the end of a ~90-minute cycle (for example after 7.5 hours rather than 8) leaves them less groggy than being woken from deep sleep. Cycles vary between people and nights, so treat it as an experiment, not a rule.
Real world
Acting out dreams
In REM sleep behaviour disorder, the normal paralysis fails and people physically act out dreams — punching or kicking. It's important medically because it is often an early warning sign of Parkinson's disease or related conditions, years before other symptoms.
Key takeaways
- Sleep cycles through N1, N2, N3 (deep slow-wave) and REM about every 90 minutes.
- REM sleep has wake-like brain waves, vivid dreams and muscle paralysis.
- Deep sleep dominates early in the night; REM lengthens toward morning.
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