Sleep calculator
Sleep runs in cycles of roughly 90 minutes, and waking at the end of one rather than in the middle is why six hours can feel better than seven. Pick a wake-up time and get bedtimes, or the other way round.
- Cycles in that night
- 5
- Time asleep
- 7 h 30
- Starting from
- 07:00
- Minutes allowed to fall asleep
- 14
- Cycle length used, minutes
- 90
| Cycles | Time | Sleep |
|---|---|---|
| 6 | 21:46 | 9 h 00 |
| 5 | 23:16 | 7 h 30 |
| 4 | 00:46 | 6 h 00 |
| 3 | 02:16 | 4 h 30 |
The formula
Bedtime = wake time − (cycles × 90 min) − time to fall asleep. Wake time = bedtime + time to fall asleep + cycles × 90 min. Five cycles, seven and a half hours, is highlighted as the usual recommendation for adults.
Worked example
To wake at 07:00, allowing 14 minutes to fall asleep: go to bed at 23:16 for five cycles, 21:46 for six, or 00:46 for four.
How to use it
- 1Enter times in 24-hour form, such as 07:00 or 23:30.
- 2Adjust the time to fall asleep to your own; 10 to 20 minutes is typical.
- 3Treat the times as candidates. Real cycles run anywhere from 70 to 120 minutes and vary through the night.
Questions people ask
How much sleep do adults need?
Seven to nine hours for most adults, by the consensus of sleep medicine bodies. Five or six cycles fall inside that; four does not, however well-timed.
Is the 90-minute cycle exact?
No. It is an average, and cycles lengthen in REM sleep towards morning. Use the times as a starting point and adjust by a quarter of an hour either way to suit yourself.
Last reviewed: 2026-10-20