The stages
- N1 — the drifting transition, a minute or two. Easy to wake from; often not remembered as sleep at all.
- N2 — light sleep, the largest share of the night. Heart rate slows, temperature falls.
- N3 — slow-wave or deep sleep. Hard to wake from. Concentrated in the first half of the night, and most associated with physical restoration and clearing of metabolic waste.
- REM — rapid eye movement, dreaming, near-paralysis of the voluntary muscles. Concentrated in the second half, and associated with memory consolidation and emotional processing.
The cycle
A full cycle runs roughly 90 minutes, though it varies between individuals and across the night. Early cycles contain most of the deep sleep; later cycles contain most of the REM. This asymmetry has practical consequences:
- Cutting sleep short from the end — a late night plus an early alarm — costs you disproportionately more REM than anything else.
- Waking in the second half disrupts REM-rich sleep, which is why maintenance insomnia has particular effects on mood and memory.
- Deep sleep declines with age, which is why older adults sleep more lightly — see sleep after 60.
Why you wake at cycle boundaries
Brief awakenings between cycles are universal — most people have several a night and do not remember them. Problems start when an awakening becomes prolonged, which is what maintenance insomnia is. The cycle boundary is the opportunity; the response determines whether it matters.
What wearables can and cannot tell you
Consumer devices estimate stages from movement and heart rate. They are reasonably good at telling sleep from wake in aggregate and poor at distinguishing specific stages — quiet wakefulness is routinely scored as light sleep. Treat stage breakdowns as entertainment rather than measurement. See orthosomnia.
Why this matters practically
- Consolidation beats duration. Six and a half hours of unbroken cycling beats eight fragmented hours — which is why sleep restriction can leave people feeling better on less.
- Waking at the end of a cycle feels easier than being woken from deep sleep, which is part of the logic behind sleep-cycle alarm apps — though their accuracy is limited by the same measurement problem.
- Alcohol suppresses REM early in the night, and the later rebound fragments sleep. See sleeping after drinking.