Health & Fitness · 睡眠周期（Sleep Cycle） · Updated 2026-10-25
What Is Sleep Cycle?
Definition
A night comprises 4-6 cycles of ~90 minutes (deep slow-wave sleep dominates early, REM later). Deep sleep handles physical repair and growth hormone; REM consolidates emotional memory. Planning in 90-minute multiples (7.5h = 5 cycles) reduces being woken mid-deep-sleep — the mechanism behind "6 hours sometimes feels better than 7." Individual cycles run 80-110 minutes; log natural wake points for 1-2 weeks to calibrate.
Formula
Sleep ≈ cycles × 90 min + sleep latency (~15 min)
Worked Example
For a 6:30 wake with 5 cycles, in bed 22:45 (asleep 23:00). If lights out slips to midnight, take 4 cycles (6h) to 6:30 rather than 7 hours ending mid-deep-sleep.
Common Misconception
"Everyone needs 8 hours" — adults need 7-9; about 1% carry short-sleeper genes thriving on 6, and consistently 9+ hours tracks with higher mortality (J-curve). "Weekend sleep-ins repay the debt" — they repay some REM but not deep-sleep debt; circadian misalignment (social jetlag) drives the lingering fatigue.
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