Sleep Calculator
Sleep runs in cycles of about 90 minutes. Waking at the end of one feels far better than waking in the middle, even when the middle gives you more sleep.
How the math works
Bedtime = wake time - (cycles x 90 minutes) - the time it takes you to fall asleep. A sleep cycle averages 90 minutes, though it ranges from 70 to 120 between people and lengthens through the night. Six cycles is 9 hours, five is 7.5.
Common questions
- What time should I go to bed?
- Whatever lands a whole number of 90 minute cycles before you must be up, plus the time it takes you to fall asleep. To wake at 7 AM, going to bed at 9:45 PM or 11:15 PM both work better than 10:30.
- Is it better to sleep 6 hours or 7?
- Seven, almost always. The cycle idea helps you pick between two nearby times, but it does not mean less sleep beats more. Six hours nightly carries a measurable cost that most people underestimate in themselves.
- Are sleep cycles really 90 minutes?
- On average. Individuals range from about 70 to 120 minutes, and cycles get longer and more REM-heavy toward morning. Treat these times as good guesses, not precision instruments.
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