How to use the Sleep Calculator
- Choose whether you have a set wake-up time or a set bedtime.
- Enter the time.
- Adjust how long you usually take to fall asleep.
- Pick one of the suggested times.
What does this tool do?
Sleep moves through cycles of light, deep and REM sleep, each lasting roughly 90 minutes. Waking at the end of a cycle, rather than in the middle of deep sleep, tends to feel easier.
The calculator counts back (or forward) in 90-minute cycles and adds the time you usually need to fall asleep.
Why use it?
- Wake up feeling less groggy.
- Plan an early start the night before.
- Simple, no app needed.
Example
To wake at 7:00 AM and allowing 15 minutes to fall asleep, good bedtimes are 9:45 PM (six cycles, 9 hours) or 11:15 PM (five cycles, 7.5 hours).
Accuracy and limits
- Cycle length varies from about 70 to 120 minutes between people and through the night. Treat the times as a guide.
- Adults generally need 7–9 hours of sleep. If you're regularly tired despite enough sleep, talk to a doctor.
Privacy
The calculation happens instantly in your browser. The numbers you enter are not sent to our servers or saved. There's no account to create and nothing to install.
Last reviewed by the M2Toolkit team.