Sleep Cycle Calculator
Alarm time in — the bedtimes that dodge mid-cycle grogginess.
Times below are lights-out times — they already include ~15 minutes to fall asleep.
How this is calculated
bedtime = wake − (cycles × 90 min + 15 min to fall asleep)
For a 06:30 alarm: six cycles puts lights-out at 21:15, five at 22:45, four at 00:15. Cycles vary between people (80–110 minutes), so treat these as starting points and adjust by feel over a week — and if the choice is between a "perfect" 4-cycle night and an imperfect 5-cycle one, take the extra sleep. Consistent wake times matter more than any arithmetic. Not medical advice.
Frequently asked questions
Why 90-minute cycles?
Sleep moves through light, deep, and REM stages in cycles averaging about 90 minutes. Waking at a cycle boundary tends to feel clearer than waking mid-deep-sleep — that grogginess is sleep inertia, and dodging it is the whole trick of this calculator.
How are the bedtimes calculated?
Count back from your alarm in 90-minute cycles, plus about 15 minutes to fall asleep. For a 06:30 alarm: six cycles suggests 21:15, five suggests 22:45, four suggests 00:15 — lights-out times, not get-into-bed times.
How many cycles do I need?
Five to six cycles (7.5–9 hours) is the adult sweet spot; four (6 hours) works occasionally, not sustainably. The calculator lists the best option first and honestly labels the short one as the compromise it is.
Is 90 minutes exact for everyone?
No — real cycles run roughly 80–110 minutes and shift through the night. Treat these times as good starting points, adjust by how you feel over a week, and keep the wake time consistent — regularity beats optimization. Not medical advice.