The 90-minute cycle
Sleep is not a flat state: you descend through light and deep sleep into REM and surface again, roughly every 90 minutes. An alarm that lands in deep sleep produces that leaden, worse-than-before-bed feeling; the same alarm twenty minutes later, at the top of a cycle, feels almost gentle. The calculator simply counts whole cycles back from your alarm.
Why the fall-asleep buffer matters
Getting into bed is not falling asleep. Most people take 10–20 minutes to drift off, which is why the bedtime shown under each option is earlier than the moment the cycles start counting. If you know you drop off instantly or you toss for half an hour, change the buffer to match.
Five or six cycles is the target
Six cycles is nine hours of sleep; five is seven and a half — the range where most adults function best. Four is a compromise for short nights; three is a salvage operation. The highlighted options are the ones worth planning around, and consistency beats optimisation: the same wake time every day trains the cycle to end on schedule.
Frequently asked questions
Are sleep cycles exactly 90 minutes?
They average 90 minutes but genuinely vary — 80 to 110 minutes between people and across the night. Treat the suggested times as good targets, not precision engineering, and adjust by how you feel over a week.
Is 6 hours of sleep enough?
Four cycles (6 h) is workable occasionally, but most adults need 7–9 hours regularly. Chronic short sleep accumulates — if you always need an alarm and always feel dreadful, the fix is an earlier bedtime, not a better-placed alarm.
Is my data stored or sent anywhere?
Nothing is uploaded. The time card is saved only in your own browser's local storage so it survives a refresh; clearing your browser data removes it.
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