Sleep cycle calculator
Sleep runs in roughly 90-minute cycles. Work backwards from your wake-up time to find bedtimes that land between cycles.
Cycle length varies by person; use this as a starting point.
How this calculator works
Sleep cycles last about 90 minutes and end with lighter sleep, which is the easiest moment to wake. If you wake mid-cycle — especially in deep sleep — you can feel groggy even after a long night. Working backwards in 90-minute blocks puts the alarm at a cycle boundary.
The calculator subtracts a buffer for the time you take to fall asleep, and shows the bedtimes for one cycle fewer and one cycle more. Most adults need 5–6 cycles (7.5–9 hours).
The formula
bedtime = wake-up time − (cycles × 90 minutes) − fall-asleep bufferWorked example
To wake at 07:00 after 5 cycles (about 7.5 hours of sleep) with a 15-minute buffer, go to bed at 23:15. Alternatives: 21:45 or 00:45.
Common mistakes
- Ignoring the fall-asleep buffer. Most people take 10–20 minutes to fall asleep; skip it and you cut a cycle short.
- Treating 90 minutes as exact. Cycle length varies between 70 and 110 minutes and changes through the night.
- Using it to justify too little sleep. Four cycles is 6 hours; most adults need more for sustained performance.
Frequently asked questions
How long is a sleep cycle?
About 90 minutes on average, ranging roughly 70–110 minutes and lengthening towards morning.
How many cycles do I need?
Most adults do best with 5–6 cycles a night, which is 7.5–9 hours including brief awakenings.
Why do I feel tired after 8 hours?
Waking in the middle of a deep-sleep cycle can feel worse than waking after fewer cycles at a light-sleep boundary.
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