Sleep Calculator

What this sleep calculator does

This sleep calculator works out the best times to go to bed or wake up so that you finish a full sleep cycle instead of being pulled out of deep sleep. Tell it when you need to wake up and it lists the bedtimes that fit. Or enter the time you are going to bed, or press Sleep now, and it lists the wake-up times. It counts in 90-minute cycles and adds the time you take to fall asleep.

How to use it

1. Pick Wake up at, Go to bed at or Sleep now.
2. Enter the time. With Sleep now the tool uses the current time on your device.
3. Choose your age group so the result is checked against the sleep range usually advised for it.
4. Adjust the minutes you take to fall asleep if 15 is not right for you. Most people need 10 to 20.
5. Read the list. Times marked Recommended give you enough total sleep for your age group.

The calculation

For a wake-up time the tool subtracts the cycles and then the time to fall asleep: bedtime = wake-up time − (cycles × 90 minutes) − minutes to fall asleep. For a bedtime it adds them: wake-up time = bedtime + minutes to fall asleep + (cycles × 90 minutes). Example: to wake at 7:00 with 15 minutes to fall asleep, six cycles (9 hours) mean going to bed at 9:45 PM, and five cycles (7.5 hours) at 11:15 PM.

How much sleep do you need?

Age group	Usual advice
Children, 6 to 13	9 to 11 hours
Teens, 14 to 17	8 to 10 hours
Adults, 18 to 64	7 to 9 hours
Older adults, 65+	7 to 8 hours

These are general ranges from sleep organisations, not medical advice. Some people feel fine at the low end and others need the high end.

Frequently asked questions

Is a sleep cycle really 90 minutes?

On average, yes, but it varies from about 80 to 110 minutes between people and from night to night. You can change the cycle length in the tool if you know yours. Our guide to the 90-minute sleep cycle explains the evidence.

Why add 15 minutes?

Because you do not fall asleep the moment you lie down. Ten to twenty minutes is typical, so 15 is a sensible default.

Will I wake up refreshed if I follow the times?

It can help, since waking from light sleep usually feels easier than waking from deep sleep. It is not a guarantee, because cycles are not perfectly regular.

How many cycles should I aim for?

Five or six for most adults, which is 7.5 or 9 hours. Four cycles is 6 hours, which is short for most people.

Is my data stored?

No. Everything is calculated in your browser and nothing is sent anywhere.

Planning your day around it? Try the time zone converter after travel, the calorie calculator, or the BMI calculator.