Sleep Calculator

Using 90-minute cycles and 15 minutes to fall asleep, the four bedtimes are 8:45 PM (6 cycles, 9.0 hours asleep), 10:15 PM (5 cycles, 7.5 hours), 11:45 PM (4 cycles, 6.0 hours) and 1:15 AM (3 cycles, 4.5 hours). Each is the wake-up time minus cycles × 90 minutes, minus your fall-asleep minutes.

7.5 hours. The calculator treats one cycle as a flat 90 minutes, so 3 cycles is 4.5 hours, 4 is 6.0 hours, 5 is 7.5 hours and 6 is 9.0 hours. Those figures are time asleep only — the minutes you spend falling asleep are added separately when working out the clock time.

Ninety minutes is an average, not a fixed number. Sleep research generally puts a full cycle somewhere in the region of 70 to 120 minutes, and it varies between people and across a single night — early cycles tend to be shorter than later ones. This tool uses a flat 90-minute cycle for every calculation, so treat the times it produces as a scheduling guide rather than a precise prediction of your sleep stages.

That is the sleep latency field — how long you take to actually fall asleep after getting into bed. It defaults to 15 minutes and accepts any whole number from 0 to 60. It is added to the total in both modes, so a bedtime result tells you when to be in bed, and a wake-up result already accounts for the time before you drop off.

The direction of the maths. "Wake up at" takes your alarm time and subtracts each cycle option to give bedtimes. "Sleep at" takes the time you are getting into bed and adds each option to give wake-up times. The four cycle counts (6, 5, 4 and 3) and the fall-asleep minutes are identical in both.

The result cards show the clock time only, with no date label — so a bedtime of 1:15 AM for a 6:00 AM alarm is the following morning, and a wake-up time can likewise roll past midnight. Times are formatted with your browser's local settings, which means you will see a 12-hour AM/PM clock or a 24-hour clock depending on your device's region.

Only for whole cycles. The tool offers 6, 5, 4 and 3 cycles, so the shortest result it produces is 4.5 hours of sleep — too long for a nap. There is no option for a partial cycle or a 20-minute nap.

Yes, it is free with no signup, and no data is stored or transmitted. The time and fall-asleep values stay in the page's memory while you use it and are gone when you close the tab — there is no network request and nothing is written to your device. It is a scheduling calculator, not a medical device; if you regularly struggle to sleep or wake unrested, that is worth raising with a doctor.