Sleep progresses through cycles of roughly 90 minutes, each containing light sleep, deep (slow-wave) sleep and REM (rapid eye movement) sleep. Waking mid-cycle — especially during deep sleep — causes grogginess (sleep inertia). Timing your alarm to the end of a complete cycle (after 7.5 hours = 5 cycles, or 9 hours = 6 cycles) means you surface naturally during lighter sleep stages. Athletes prioritising recovery aim for at least 8–9 hours (5–6 full cycles).
Key points
- Cycles last roughly 90 minutes
- Stages: light, deep and REM
- Wake at cycle end to avoid grogginess
- 5 cycles (7.5 hrs) suits most adults