Sleep Calculator
Works out when to go to bed for a set wake-up time, or when to wake for a set bedtime.
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The formula
bedtime = wake time − (cycles × 90 minutes) − time to fall asleep wake time = bedtime + time to fall asleep + (cycles × 90 minutes)
Times are 24-hour and wrap past midnight. The 90 minutes is an average: real cycles commonly run from about 70 to 120 minutes.
What to watch out for
Sleep runs in cycles of light sleep, deep sleep and REM, averaging about 90 minutes. Timing your alarm for the end of a cycle may leave you less groggy, though the evidence for this is limited and cycles are not regular enough to hit exactly.
Getting enough sleep matters more than the timing. Adults generally need 7–9 hours. If you often sleep badly or feel unrefreshed, it is worth raising with a doctor.
Frequently Asked Questions
What time should I go to bed to wake at 07:00?
With 15 minutes to fall asleep: 23:15 for 5 cycles, or 21:45 for 6.
When will I wake if I go to bed at 23:30?
With 15 minutes to fall asleep: 07:15 after 5 cycles, or 08:45 after 6.
Is it always 90 minutes per cycle?
No. That is an average. Cycles vary between people and across the night, with more deep sleep early on and more REM towards morning.