Wake up refreshed by sleeping in complete 90-minute cycles
Sleep happens in 90-minute cycles. Waking at the end of a cycle feels natural and refreshed. The calculator adds multiples of 90 minutes to your bedtime (plus 14 min to fall asleep) to show the ideal wake times.
Most adults need 5–6 sleep cycles per night (7.5–9 hours). The minimum is 4 cycles (6 hours). 3 cycles or fewer will leave you tired.
8 hours doesn't align to 90-min cycles — it falls mid-cycle. Try 7.5 hours (5 cycles) or 9 hours (6 cycles) instead.
A 90-minute block of light sleep, deep sleep, and REM (dream) sleep. Your brain repeats this all night. Waking mid-cycle causes "sleep inertia" — that groggy, heavy feeling.
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