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Sleep cycles average 90 minutes (range: 80–120 min). Waking mid-cycle causes sleep inertia (grogginess). Optimal wake times align with cycle completions: 5 cycles = 7.5 hours, 6 cycles = 9 hours. Most adults need 7–9 hours. The 20-minute nap (stages 1–2 only) avoids deep sleep and grogginess. Consistent sleep timing (even weekends) is the single most effective sleep hygiene practice.
90 min cycles + ~15 min to fall asleep.
Try sleeping by 21:45