How Sleep Cycles Work
Each night your brain cycles through a predictable rhythm. A single sleep cycle lasts approximately 90 minutes and consists of four stages: N1 (light sleep, ~5 min), N2 (deeper light sleep, ~20 min), N3 (slow-wave deep sleep, ~50 min), and REM (rapid eye movement, ~15 min). The ratio shifts across cycles — early cycles contain more deep sleep; later cycles contain more REM.
Waking during N3 (deep sleep) causes sleep inertia — the grogginess that makes you feel worse than if you'd slept less. By timing your alarm to land between cycles, you wake naturally refreshed regardless of total sleep time.
The Colour-Coded Cycle Timeline
Each 90-minute bar in the timeline above the result cards is divided proportionally: the first cycle is ~55% deep sleep; by cycle 5–6 it's ~80% REM. The division is an illustration based on average research findings.
Recommended Sleep by Age
| Age Group | Hours | Cycles (90 min) | Notes |
|---|---|---|---|
| 0–3 months | 14–17 h | 9–11 | Multiple naps throughout the day |
| 4–11 months | 12–15 h | 8–10 | Consolidating to 2–3 naps per day |
| 1–2 years | 11–14 h | 7–9 | One afternoon nap typically remains |
| 3–5 years | 10–13 h | 7–9 | Napping often ceases by age 5 |
| 6–13 years | 9–11 h | 6–7 | Consistent bedtime critical |
| 14–17 years | 8–10 h | 5–7 | Circadian shift pushes bedtime later |
| 18–64 years | 7–9 h | 5–6 | 6 cycles (9 h) optimal for most adults |
| 65+ years | 7–8 h | 5–6 | More fragmented; napping acceptable |
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