The stages at a glance
Experts distinguish non-REM sleep, with three stages, and REM sleep. Apps usually group this into light sleep, deep sleep and REM.
| Stage | What happens |
|---|---|
| Light sleep (N1, N2) | Transition from being awake; heart rate and breathing slow down and body temperature drops. |
| Deep sleep (N3) | The deepest stage, the hardest to wake up from; important for physical recovery. |
| REM sleep | Rapid eye movements and vivid dreams; the brain is very active while the muscles are relaxed. |
The sleep cycle
A night consists of several cycles of roughly 90 minutes each. Early in the night the share of deep sleep is higher, and toward morning the REM stages get longer. Short awake phases between cycles are normal – you don’t remember most of them.
Why the wake-up time matters
If you are woken from deep sleep, you often feel groggy. Waking up from light sleep is usually easier. Smart alarms build on that: they wake you within a time window during a lighter stage.