Sleep Calculator

You enter the time you want to wake up. The tool counts back a whole number of 90-minute sleep cycles, then takes off another 15 minutes for falling asleep:

bedtime = wake time − (cycles × 90 minutes) − 15 minutes

It does this for 6, 5, 4 and 3 cycles. For the default wake time of 07:00:

The six-cycle card is marked "best". That label goes to the longest option every time; it doesn't take your age or routine into account. The times appear in your device's 12- or 24-hour format.

The hours on each card are sleep time only. Time in bed is 15 minutes longer, because the tool assumes that is how long it takes you to drop off. Five cycles means 7.5 hours of sleep but 7 hours 45 minutes between lying down and the alarm.

The results move with the wake time, always in 90-minute steps. Wake at 06:00 and the options are 8:45 pm, 10:15 pm, 11:45 pm and 1:15 am. If none of them fits your evening, pick the nearest one rather than splitting a cycle down the middle, since lining up with the end of a cycle is the whole point of the method.

Both numbers in the formula are fixed: every cycle is 90 minutes and falling asleep always takes 15. Real nights vary, so treat the times as estimates rather than a schedule to follow to the minute.

You can correct the fall-asleep part yourself:

• If you usually lie awake for about 30 minutes, go to bed 15 minutes earlier than the card says.
• If you fall asleep almost at once, you can go to bed up to 15 minutes later.

The cycle length can't be adjusted, and the tool has no way to measure your own. The 90 minutes is one figure applied to everyone.

The tool only works backwards from a wake time. To go forwards from a bedtime, add the same pieces. Lying down at 23:00, add 15 minutes, then 90 minutes per cycle: 4 cycles ends at 5:15 am, 5 cycles at 6:45 am and 6 cycles at 8:15 am. Set the alarm for one of those.

Shift workers can use the tool as it is, because the wake time can be any hour. Waking at 15:00 after a night shift gives bedtimes of 5:45 am, 7:15 am, 8:45 am or 10:15 am.