Sleep Cycle Calculator
Bedtime is the lever you control. Give the calculator your wake-up time — or the moment you're about to close your eyes — and it walks the 90-minute cycles backwards and forwards from there.
Five to six cycles is the recommended zone; four is workable, three is a debt you'll feel. The clock is a guide, not a contract — if you're sleepy at cycle four, sleep.
Common questions
Is 90 minutes the exact length of a sleep cycle?
No — it's the average. Real cycles run about 80–110 minutes and lengthen through the night. The calculator uses 90 because it's the honest middle, and because waking mid-cycle at the wrong point matters more than the minutes being perfect.
Does waking at the end of a cycle really work?
Directionally, yes: alarm-at-cycle-end is the idea behind every sleepytime calculator, and waking from lighter sleep feels easier than being torn out of deep sleep. The effect is real but modest — total sleep hours still matter more than the math.
How it's computed
Sleep runs in cycles averaging 90 minutes (light → deep → REM), and waking at a cycle boundary beats waking mid-deep-sleep. The calculator subtracts your fall-asleep latency, then steps whole cycles from your anchor time. It's the same arithmetic every sleepytime chart uses — with the numbers shown instead of hidden.
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