Sleep Calculator

What your result means

Count back from your wake time in whole cycles, then subtract the minutes you take to fall asleep. Wake at 7:00 AM with 15 minutes to fall asleep and 90-minute cycles, and the bedtimes are 9:45 PM for six cycles, 11:15 PM for five and 12:45 AM for four. From a bedtime the sum runs forward: add the minutes to fall asleep, then the cycles. This sleep cycle calculator only does clock arithmetic on two values you can change. The three rows are shown equally; none is marked as better, and the times are not a prescription for how long anyone should sleep.

How to use the Sleep Calculator

1. Choose Wake time, Bedtime or Sleep now. Wake time lists bedtimes; Bedtime and Sleep now list wake times.
2. Enter the time. Sleep now needs none, because it reads your device’s clock.
3. Change the two settings if you like: minutes to fall asleep and cycle length. Both start from a value you can change.
4. Read the rows for 4, 5 and 6 cycles. Each gives a clock time and the time asleep. Show times as switches between 12-hour and 24-hour and starts from your browser’s setting.
5. Press Log this night to add the night to your calendar, or select a day on the calendar and press Add night or Edit night.

Formula and worked example

bedtime    = wake time − (cycles × cycle length) − minutes to fall asleepwake time  = bedtime + minutes to fall asleep + (cycles × cycle length)cycles for a logged night = round((minutes in bed − minutes to fall asleep) ÷ cycle length), never below 0Times wrap past midnight on a 24-hour clock.

Worked example: to wake at 7:00 AM with 15 minutes to fall asleep and 90-minute cycles, 6 cycles is 9 h asleep, so bedtime 9:45 PM; 5 cycles is 7 h 30 min asleep, so bedtime 11:15 PM; 4 cycles is 6 h asleep, so bedtime 12:45 AM. Each answer is the wake time minus the sleep, minus the 15 minutes. These are clock sums, not a prescription for how long anyone should sleep.

How long is one sleep cycle

Sleep has stages: non-REM sleep and REM sleep. Over a night the stages repeat in a cycle. The National Heart, Lung, and Blood Institute (NHLBI) says that a cycle repeats about every 80 to 100 minutes and that sleep usually has four to six cycles a night.

This page starts from 90 minutes, the middle of that range, and lets you change it from 80 to 110 minutes. 90 is a starting value for the arithmetic, not a measurement of your own sleep. The cycle lengths of real nights vary, and nothing on this page can tell you yours.

About the daily log

The daily log is a month calendar under the calculator. Press Log this night, or tap a day, then enter the time you went to bed and the time you got up. Each logged day shows the time asleep and the number of cycles, always counted with 15 minutes to fall asleep and 90-minute cycles, whatever the settings above are.

You can set your own target in cycles per night, from 1 to 8. A night that reaches it shows a smiling face, a night below it shows a neutral face, and with no target set there is no face. The face only compares a night with the number you chose. It is not a judgement of the night, and the page never describes a night as better or worse than another.

The switch Remember my log on this device saves the log in this browser’s own storage so it is there next visit. It starts on and is also in the Log a night form, but nothing is saved just by opening the page: the log is saved when you save a night or change a log setting. Turn the switch off before you save and the log is held in the page only until you close or reload it. The log is not sent to our servers, not included in our analytics and not put in the page address. No account is needed.

Browsers can clear saved site data, so export a CSV file now and then. Import CSV brings a file back on this device. Clear log deletes every night, removes your target and switches Remember off. The log holds up to 366 nights; past that, the oldest is dropped.

Wake times by cycle count

Each wake time is the bedtime plus 15 minutes to fall asleep plus the cycles at 90 minutes each (4 cycles = 6 h, 5 cycles = 7 h 30 min, 6 cycles = 9 h). Read it the other way to find a bedtime for a wake time. Change the two starting values in the calculator to match your own.

Hours of sleep by age

Recommended daily sleep by age group, as published by the CDC (About Sleep, page dated May 15, 2024). A published reference range. This page does not judge any person’s sleep.

Assumptions and limits

• The 15 minutes to fall asleep and the 90-minute cycle are editable starting values, not measurements of you. Change them and every time updates.
• The 12-hour or 24-hour display starts from your browser’s setting and is not saved; switch it with Show times as.
• Sleep now uses your device’s clock, read when you change a setting or come back to the page. It is not stored.
• Times are wall-clock times. A daylight-saving change during the night is not modelled.
• This is scheduling arithmetic, not medical advice. No figure here diagnoses anything, and the page does not know what you need.

Frequently asked questions

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