How this calculator works
The 90-minute-cycle scheduling method estimates bedtime for healthy adults by subtracting cycles and sleep latency from wake time. Actual cycles vary, often around 80 to 110 minutes, and cannot be predicted from a clock alone. Five cycles gives 7.5 hours and six gives 9 hours. Three or four cycles is short sleep, not an adequate-rest recommendation. The result is an estimate for information only and is not medical advice.
Formula
Bedtime minutes = (wake minutes - 90 x cycles - fall-asleep minutes) modulo 1440. Time asleep = 90 x cycles minutes.
Worked example
Six assumed cycles give 9 hours asleep. With a 06:00 wake time and 15 minutes of latency, bedtime is 20:45 the previous evening.
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Frequently asked questions
Are sleep cycles exactly 90 minutes?
No. Cycle length varies between people and during the night. This is only a schedule estimate.
Does time to fall asleep count as sleep?
No. Latency moves bedtime earlier but does not increase the calculated time asleep.
Does choosing six cycles always mean nine hours asleep?
In this model, yes. Time asleep is exactly 90 minutes times the number of cycles, so six cycles is 9 hours whatever the wake time.