Sleep cycle calculator: find the best bedtime or wake time so you complete full 90-minute sleep cycles and wake up refreshed. Free, private, no signup.
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General estimate for planning only — not medical advice. Consult a doctor about your specific situation.
Calculation details
Every figure above is calculated locally in your browser from the assumptions shown. No inputs are sent anywhere. See the methodology section below for how this tool works.
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Assumptions to Verify
These tools provide general estimates for planning and education only.
They are not medical advice and not a substitute for a qualified healthcare professional.
Always consult your doctor or another healthcare provider about your specific situation.
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Worked example: waking at 7:00 AM
Set the wake time to 7:00 AM, which is 420 minutes after midnight. The calculator subtracts 15 minutes for sleep onset, then counts back in 90-minute cycles: 4 cycles is 360 minutes, 5 cycles is 450, and 6 cycles is 540.
For 6 cycles: 420 - 540 - 15 = -135. Since that lands before midnight, 1,440 minutes (a full day) is added, giving 1,305 minutes, or 9:45 PM. For 5 cycles: 420 - 450 - 15 = -45, which becomes 1,395 minutes, or 11:15 PM. For 4 cycles: 420 - 360 - 15 = 45, so 12:45 AM.
The middle option, 11:15 PM, is shown as the hero result because 5 cycles (7.5 hours plus the 15-minute buffer) sits closest to the 7 to 9 hours most adults need. Choosing 9:45 PM gives 8 hours asleep if you nod off in 15 minutes, and 12:45 AM gives 5 hours for short nights.
What the 90-minute figure does not capture
The 90-minute cycle is an average; individual cycles run roughly 80 to 120 minutes, and they lengthen through the night, with deep sleep concentrated in the first two cycles and REM dominating later ones. If your true cycle is 105 minutes, a 5-cycle target computed at 90 minutes lands about 75 minutes early, and you would wake mid-cycle anyway.
The 15-minute onset allowance is also a guess. A 2020 fitbit-style dataset analysis of sleep latency in healthy adults puts typical onset between 10 and 20 minutes, but stress, screens, or caffeine can push it past 45 minutes, shifting every bedtime by that same amount.
Treat the outputs as a starting grid, not a schedule: pick the nearest option, track how groggy you feel for a week, and adjust by 15 to 20 minutes per night rather than recomputing from scratch.
