Sleep Calculator
Go to bed at a time that lets you wake at the end of a sleep cycle.
Your numbers
Your results
Sleep Calculator by TheCalcTools
How the Sleep Calculator works
Enter the time you need to wake up and how long you usually take to fall asleep.
Sleep runs in cycles of roughly 90 minutes, which the tool counts back from your wake time.
Pick a bedtime; five to six cycles suits most adults.
The formula
Worked example
Using the default values shown in the calculator:
| Wake-up hour (0–23) | 7 |
|---|---|
| Wake-up minute | 0 |
| Minutes to fall asleep | 15 |
- Waking at 07:00 with 15 minutes to fall asleep: 5 cycles → bed at 23:15.
- 6 cycles → 21:45; 4 cycles → 00:45.
Result:
| 6 cycles (9 hours) | 21:45 |
|---|---|
| 5 cycles (7.5 hours) | 23:15 |
| 4 cycles (6 hours) | 00:45 |
| 3 cycles (4.5 hours) | 02:15 |
Frequently asked questions
Are cycles exactly 90 minutes?
No. Cycle length varies between about 80 and 110 minutes and from night to night, so treat the result as a guide.
How much sleep do adults need?
Most adults need seven to nine hours. Needs vary between individuals.
Sources and further reading
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This calculator gives general estimates for information and education. It is not medical advice and does not diagnose anything. Talk to a qualified health professional about your own situation. See our disclaimer.