How to use the Sleep Calculator
The sleep calculator suggests bedtimes or wake-up times that line up with the end of a 90-minute sleep cycle, so you are more likely to wake up feeling refreshed. Choose a wake-up time to find your bedtime, a bedtime to find your wake-up time, or press the sleep-now option to start from the current time.
What you need to enter
- I want to
- Time
- Minutes to fall asleep
- Clock format
The formula
Sleep moves through repeating cycles of about 90 minutes. Bedtime = wake-up time − (number of cycles × 90 minutes) − the minutes you take to fall asleep. Wake-up time = bedtime + minutes to fall asleep + (cycles × 90 minutes). Most adults do well on 5 to 6 cycles, which is 7.5 to 9 hours.
Worked example
Using the default values in the calculator:
- I want to: Wake up at… (find my bedtime)
- Time: 07:00
- Minutes to fall asleep: 15
- Clock format: 12-hour (AM/PM)
| Go to bed at 9:45 PM | 6 cycles · 9 hours of sleep · recommended |
| Go to bed at 11:15 PM | 5 cycles · 7.5 hours of sleep · recommended |
| Go to bed at 12:45 AM | 4 cycles · 6 hours of sleep · minimum |
| Go to bed at 2:15 AM | 3 cycles · 4.5 hours of sleep · short |
Tips for better results
- Change the minutes to fall asleep to match your own habits; 15 is an average.
- Aim for 5 or 6 full cycles most nights, and treat 4 cycles as a minimum for occasional short nights.
- Keep a steady bedtime and wake-up time, because a regular schedule helps more than any single calculation.
Common mistakes to avoid
- Expecting the cycle length to be exactly 90 minutes every night; it varies from person to person and across the night.
- Counting from when you get into bed instead of from when you actually fall asleep.
- Using the tool in place of medical advice for ongoing sleep problems.
Please note: Results are estimates for general use. For safety-critical work such as electrical or building projects, check with a qualified professional and local codes. See our Disclaimer.