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Sleep comes in ~90-minute cycles. Waking at the end of a cycle beats waking mid-cycle, even with fewer total hours. Pick your mode:
Educational timing tool, not medical advice. Cycle length averages 90 minutes but varies by person.
Bedtime = wake time − (cycles × 90 min + fall-asleep time). Five cycles (7.5 h) hits the recommended 7+ hours for most adults; six (9 h) is the full-rest option. Waking mid-cycle causes sleep inertia — the grogginess that coffee barely fixes.
8 hours = 5.3 cycles — you likely woke mid-cycle. 7.5 h (5 cycles) or 9 h (6 cycles) ends at a cycle boundary.
Four cycles is below the 7-hour minimum most sleep researchers recommend. Fine occasionally, not as a routine.
It is a population average (range ~80–110 min). Treat the times as a window and shift ±15 min if you consistently wake groggy.