Sleep calculator
Sleep calculator — bedtimes and wake times by sleep cycle
Work back from the alarm, or forward from the pillow, in whole 90-minute sleep cycles — with the fifteen minutes it takes to fall asleep already counted in. Free, instant, no sign-up.
A time on a page is the easy part. The coach works on the evening that has to end at it.
What a sleep cycle calculator is for
Sleep runs in cycles, and waking at the end of one, in light sleep, feels better than being pulled out of deep sleep partway through. That is why six hours can leave you sharper than seven. This page takes the one time you already know — the alarm you cannot move, or the moment you are getting into bed — and lists the times at the other end of the night that land on a whole cycle.
How this calculator works
A cycle is taken as 90 minutes, and four, five and six of them are offered: four is a short night, six is a full one. Falling asleep is counted separately from sleeping. Working back from a 07:00 alarm through five cycles lands at 23:30 asleep, so the bedtime offered is 23:15 — fifteen minutes earlier. Calculators that skip that step are systematically a quarter of an hour optimistic. Going the other way, the time you enter is when you get into bed, so both the sleep and the fifteen minutes are added on.
The fifteen minutes most calculators skip
Fifteen minutes is the conventional planning figure for falling asleep, and it sits inside the 10 to 20 minutes considered normal. The edges are informative in their own right: consistently dropping off in under five minutes suggests sleep deprivation rather than a talent for sleeping, and consistently taking more than thirty is the threshold used in insomnia screening.
How much sleep the recommendation asks for
The range shown for your age comes from the National Sleep Foundation's 2015 consensus panel (Hirshkowitz et al., Sleep Health): 9 to 11 hours up to age 13, 8 to 10 through 17, 7 to 9 from 18 to 64, and 7 to 8 from 65. The panel also published a wider "may be appropriate" band on either side of those figures, which this page deliberately leaves out — a calculator that widens its own target until everybody passes is not telling anyone anything. It is also why the four-cycle option is marked as the short night it is rather than offered as an equal choice.
Where ninety minutes breaks down
Ninety minutes is a population mean being used as if it were a personal constant. Real cycles run roughly 70 to 120 minutes, they lengthen across the night as REM takes a bigger share, and they differ between people and between nights. So treat every time on this page as a candidate about a quarter of an hour wide, not as an appointment. The precision in the popular version of this idea is not in the biology.