Sleep Calculator
90-minute cycle math for bedtime or wake time, with a fall-asleep buffer. Click any i for detail.
Schedule
Mode
i
Mode
Plan bedtime from wake, or wake from bedtime. |
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|---|---|
Wake or bedtime
i
Anchor
HH:MM (24-hour). |
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Sleep cycles
i
Cycles
Each cycle ≈ 90 minutes; 5–6 common. |
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Fall-asleep buffer (min)
i
Buffer
Time to fall asleep. |
How to use this calculator
- Choose bedtime or wake mode.
- Enter the anchor time as HH:MM.
- Set cycles and fall-asleep buffer.
- Read the suggestion and alternate cycle lengths.
Results explained
Many popular sleep timers assume ~90-minute cycles. Bedtime ≈ wake − (cycles×90) − buffer. Real sleep architecture varies by person and age.
Quick reference: 90-minute cycles
sleep span = cycles × 90 minutes (+ buffer to fall asleep).
| Item | Detail |
|---|---|
| 5 cycles | 7.5 hours asleep |
| 6 cycles | 9 hours asleep |
| Buffer | Time to fall asleep |
| Naps | Often 1 cycle |
Educational schedule aid — not a sleep-disorder diagnosis.
How the estimate is built
span = cycles×90; bedtime = wake − span − buffer; wake = bedtime + buffer + span.
Example scenario
Wake 7:00, 5 cycles, 15 min buffer → bedtime 10:15 PM.
FAQ
Are cycles exactly 90 minutes?
No — averages vary; this is a planning heuristic.
AM/PM input?
Use 24-hour HH:MM (e.g. 19:30).
Caffeine?
See the caffeine half-life calculator.
Insomnia?
Talk to a clinician — timers don’t treat disorders.