What it does
Sleep comes in cycles of light, deep and dream sleep that last about 90 minutes each. Waking at the end of a cycle, when sleep is light, often feels easier than an alarm in the middle of deep sleep. This calculator counts back from your wake-up time, or forward from your bedtime, in whole cycles, and adds the time it takes to fall asleep. A table shows how many hours of sleep experts recommend for each age.
It is a planning aid, not medical advice. If you are planning around a pregnancy or your cycle, see the due date and the ovulation calendar.
How to use it
- Pick a tab: I wake up at, I go to bed at, or Going to bed now.
- Set the time. The results update straight away.
- Choose one of the three times. The highlighted ones (5 or 6 cycles) suit most adults.
What each setting does
- Wake-up time / Bedtime: the time you know. Hidden in “Going to bed now”, which uses your device’s clock.
- Minutes to fall asleep: added before the first cycle. 15 minutes is typical; change it if you fall asleep faster or slower.
- Cycle length: 90 minutes by default. Real cycles run from about 70 to 120 minutes, so adjust it if a time works better for you.
- Clock: shows times in 24-hour or 12-hour (AM/PM) style.
- Reset: back to the example.
Worked example
You need to wake at 07:00. Six cycles are 9 hours, plus 15 minutes to fall asleep, so go to bed at 21:45. Five cycles (7 h 30 min) give 23:15, and four cycles (6 h) give 00:45. In 12-hour style the first one reads 9:45 PM.
FAQs
How much sleep do adults need? At least 7 hours a night, by the American Academy of Sleep Medicine. The National Sleep Foundation says 7 to 9 for adults and 7 to 8 from age 65.
Is the 90-minute rule exact? No. It is an average. Use the times as a starting point and change the cycle length to suit you.
Do naps count for children? Yes. The ranges for children under 6 include naps.
Where the numbers come from
- Paruthi S. et al. (2016), Recommended Amount of Sleep for Pediatric Populations, American Academy of Sleep Medicine consensus, J Clin Sleep Med 12(6) (checked 2026-11-10).
- Watson N.F. et al. (2015), Recommended Amount of Sleep for a Healthy Adult, AASM and Sleep Research Society, Sleep 38(6): 7 hours or more (checked 2026-11-10).
- Hirshkowitz M. et al. (2015), National Sleep Foundation's sleep time duration recommendations, Sleep Health 1(1): adults 7 to 9 hours, 65 and over 7 to 8 (checked 2026-11-10).
- The 90-minute cycle and 15 minutes to fall asleep are common rules of thumb, editable on the page; fibub's own working.
Standard scientific values, such as constants, atomic weights and melting points, are facts found in every textbook; we name where ours were checked. If you are an author or publisher and want a value, a credit or a table changed or removed, tell us and we will act quickly.
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Last updated 2026-11-10.