How this calculator works
This healthy-adult planning estimate subtracts assumed 90-minute cycles and sleep latency from wake time, wrapping across midnight. Real cycles vary, often around 80 to 110 minutes; this cannot predict a sleep stage or guarantee an easier awakening. Five or six cycles give 7.5 or 9 hours, within the usual adult 7 to 9 hour range. Three or four cycles are short sleep and should not be treated as sufficient rest. 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
For a 06:00 wake time and 15 minutes to fall asleep, five cycles give bedtime 22:15 and 7 hours 30 minutes asleep; six give 20:45 and 9 hours.
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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.
What happens when the calculated bedtime is before midnight?
The result wraps around the 24-hour clock, so a bedtime earlier than 00:00 is shown as a time in the previous evening.