Sleep Cycle Calculator
The best times to fall asleep or wake up, aligned to 90-minute sleep cycles — wake between cycles, not inside one.
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Includes ~15 min to fall asleep. Green options are ideal (5-6 cycles).
Why 90 Minutes Matters
Sleep runs in ~90-minute cycles from light sleep through deep sleep into REM. An alarm that fires mid-cycle — especially during deep sleep — produces that groggy, hit-by-a-truck feeling (sleep inertia), while waking between cycles feels dramatically lighter even on the same total hours. The calculator counts back from your wake-up time in whole cycles, adding ~15 minutes to actually fall asleep.
Five to six cycles (7.5-9 h) is the adult sweet spot; four (6 h) works occasionally, not chronically. The cycle length varies ±10 minutes by person — if the suggested times still feel rough, nudge your bedtime in 15-minute steps for a week and keep what works.
Frequently Asked Questions
Is 6 hours of aligned sleep better than 8 unaligned?
No — total sleep matters more than alignment. Alignment optimizes how you feel at the alarm, not whether you got enough. Aim for 5-6 full cycles most nights.
Do naps follow the same rule?
Short naps should stay under ~25 minutes (before deep sleep begins) or go a full 90-minute cycle. The 45-60 minute nap wakes you from deep sleep — the worst spot.
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Sleep Cycle Calculator