Sleep Calculator
Sleep cycles are assumed to last 90 minutes, with 15 minutes to fall asleep. To wake at 7:00 AM, go to bed at 9:45 PM, 11:15 PM, 12:45 AM, 2:15 AM.
How to use
- Choose I need to wake up at, then set your wake-up time. You get four bedtimes, from 9 hours of sleep down to 4.5 hours.
- Or choose Go to bed now to see when to set your alarm if you lie down at the time shown, which starts as the current time and can be changed.
- Or choose Hours of sleep, enter the time you go to bed and the time you wake up, and read how many hours and sleep cycles that is. A wake-up time earlier than your bedtime counts as the next morning.
- Switch between 12-hour and 24-hour clock if you prefer.
- Treat the times as a starting point. Pick the one that fits your day and adjust after a few nights.
How it works
The estimate assumes a sleep cycle of 90 minutes and about 15 minutes to fall asleep. For a wake-up time, each bedtime is the wake-up time minus 15 minutes minus a whole number of 90-minute cycles. Six cycles is 9 hours of sleep, five is 7.5 hours, four is 6 hours and three is 4.5 hours.
For example, to wake at 7:00 AM after five cycles, subtract 7.5 hours and 15 minutes, which gives 11:15 PM. In Go to bed now mode the same sum runs forward: bedtime plus 15 minutes plus the cycles.
Times that cross midnight wrap around the clock, and a note in brackets tells you when a time falls on the previous day or the next day.
These are averages. Real sleep cycles run from about 70 to 120 minutes, change through the night and differ between people, and falling asleep can take longer or shorter than 15 minutes. This is not medical advice. If you are tired all the time or have trouble sleeping, talk to a doctor.
Sleep cycles at a glance
| Cycles | Time asleep | Time in bed with 15 min to fall asleep |
|---|---|---|
| 3 | 4.5 hours | 4 hours 45 minutes |
| 4 | 6 hours | 6 hours 15 minutes |
| 5 | 7.5 hours | 7 hours 45 minutes |
| 6 | 9 hours | 9 hours 15 minutes |
FAQ
Why 90 minutes?
Sleep moves through lighter and deeper stages in repeating cycles that average about 90 minutes. The idea is to wake near the end of a cycle rather than in the middle of deep sleep, which some people find leaves them less groggy.
How much sleep does an adult need?
Most adults do best with 7 to 9 hours, which is why the 5 and 6 cycle options are the usual targets. Needs vary from person to person.
Does it matter if I fall asleep faster or slower than 15 minutes?
Yes. If you usually take 30 minutes, go to bed 15 minutes earlier than the times shown. If you fall asleep at once, you can go a little later.
Is this accurate?
It is a rough guide based on averages. Your own cycle length is not exactly 90 minutes and it is not the same every night, so treat the times as estimates.
How many hours of sleep is 10 PM to 3 AM?
From 10 PM to 3 AM is 5 hours in bed. After about 15 minutes to fall asleep that leaves 4 hours 45 minutes, which is 3 full 90-minute cycles. Choose Hours of sleep, enter 22:00 and 03:00, and the tool does the midnight crossover for you.
How many hours of sleep is 11 PM to 7 AM?
From 11 PM to 7 AM is 8 hours in bed, or about 5 full 90-minute cycles once you allow 15 minutes to fall asleep. That is inside the 7 to 9 hours most adults aim for.
Is my time sent anywhere?
No. The times are calculated in your browser and nothing is sent to a server.