Stage 1: Light sleep onset
Lasts 1–7 minutes. Easily awakened. Heart rate and breathing begin to slow.
Stage 2: True light sleep
Body temperature drops further. Comprises about 50% of total sleep time.
Stage 3: Deep (slow-wave) sleep
Critical for physical recovery and immune function. Concentrated in the first half of the night.
Stage 4: REM sleep
Where most dreaming occurs. Crucial for memory consolidation and emotional regulation.
Why 90 minutes matters
Each cycle averages 90 minutes. Waking at the end of a cycle — not the middle — prevents grogginess.