Sleep Calculator
Sleep cycle calculator: what time to go to bed for a given wake-up time, or when to wake if you go to bed now, in 90-minute cycles plus time to fall asleep.
Results
Assumptions and formula
Time asleep is cycle length multiplied by cycle count. Add time to fall asleep to find time in bed. Subtract that duration from a wake-up time, or add it to a bedtime. A day offset of −1 means the previous day; +1 means the next.
Real cycles vary; NIH describes cycles around 80–100 minutes. This is clock arithmetic, not a way to guarantee refreshed waking. Use a sleep duration appropriate to you. Local clock only: daylight-saving and timezone changes are not modelled.
Worked example
For a 07:00 wake-up, five assumed 90-minute cycles plus 15 minutes to fall asleep means bedtime 23:15 on the previous day: 450 minutes asleep and 465 minutes in bed.
Frequently asked questions
What time should I go to bed to wake up at 6 am?
Counting back in 90-minute cycles and allowing about 15 minutes to fall asleep: 8:45 pm for six cycles (nine hours), 10:15 pm for five (seven and a half hours) or 11:45 pm for four (six hours). Adults generally need seven or more hours, so the first two are the realistic choices. The 90-minute cycle is an average, not a rule; individual cycles run from about 70 to 120 minutes and lengthen through the night, so treat the list as good bedtimes to aim for rather than exact times, and adjust by how you feel on waking. A sleep schedule calculator like this one is only a starting point for the schedule itself; keeping the same wake-up time seven days a week does more than any bedtime arithmetic. Reverse the direction and the same page is a wake up time calculator: go to bed now, and it lists the wake times that land at the end of a cycle. Either way the bedtime calculator is answering the same question people type at midnight, when should I go to bed, with a short list rather than a lecture.
Will this stop me waking during deep sleep?
No. Sleep stages vary through the night and between people. Fixed cycle arithmetic cannot predict your actual sleep stage when an alarm rings.
Does this handle daylight-saving changes?
No. It uses a local wall clock without a date or timezone. Adjust manually if the night crosses a clock change.
Sources
Sources reviewed 15 September 2026: checked against their current editions on that date.
This page is an educational estimate and is not medical advice. Follow individual guidance from your clinician.