Find your ideal bedtime or wake-up time based on 90-minute sleep cycles. Waking between cycles — not mid-cycle — prevents morning grogginess.
A complete sleep cycle lasts approximately 90 minutes and consists of four stages: N1 (light sleep, 1–7 minutes), N2 (intermediate sleep, 10–25 minutes), N3 (deep/slow-wave sleep, 20–40 minutes), and REM (Rapid Eye Movement sleep, 10–60 minutes). Most adults complete 4–6 cycles per night, totalling 6–9 hours. The proportion of deep sleep is highest in the first two cycles; REM sleep dominates cycles 4–6.
Sleep inertia — the grogginess and confusion immediately after waking — is caused by interrupting a sleep cycle mid-phase, particularly during deep sleep (N3) or REM. When you wake naturally at the end of a cycle during the light N1 or N2 phase, your brain is already in a near-waking state and the transition is smooth. This is why the same 7.5 hours of sleep with good cycle alignment feels more refreshing than 8 hours cut mid-cycle.
| Age Group | Recommended Sleep | Cycles |
|---|---|---|
| Teenagers (13–18) | 8–10 hours | 5–6 cycles |
| Adults (18–64) | 7–9 hours | 5–6 cycles |
| Older adults (65+) | 7–8 hours | 4–5 cycles |