Sleep Calculator
Find the best bedtime or wake-up time based on 90-minute sleep cycles so you wake up feeling rested. Free.
Independently verified for accuracy
Calculator by Toolsloft ↗- Bedtime, 6 cycles (~9h)
- 21:45
- Bedtime, 5 cycles (~7.5h)
- 23:15
- Bedtime, 4 cycles (~6h)
- 00:45
This sleep calculator suggests the best times to go to bed or wake up so you finish a full sleep cycle instead of waking mid-cycle. Enter the time you need to wake up and it works backward, or enter a bedtime and it works forward. It plans around 90-minute cycles plus a short buffer to fall asleep.
How this is calculated
Sleep moves through roughly 90-minute cycles, and waking at the end of a cycle tends to feel less groggy than waking in the middle. The calculator adds 15 minutes to fall asleep, then steps in 90-minute blocks to suggest times for 6, 5, and 4 complete cycles (about 9, 7.5, and 6 hours). This cycle model is a widely used sleep-hygiene heuristic, not a medical diagnosis.
How to use
- Choose whether you know your wake-up time or your bedtime.
- Enter that time in 24-hour HH:MM format.
- Read the suggested times for 6, 5, and 4 full sleep cycles.
Examples
- Wake at 07:00:
bed at 21:45 (6 cycles) or 23:15 (5 cycles) - Bed at 22:30:
wake at 07:45 (6 cycles) or 06:15 (5 cycles)
FAQ
- Why 90-minute sleep cycles?
- A full sleep cycle averages about 90 minutes as you move through light, deep, and REM sleep. Waking at the end of a cycle, rather than in the middle of deep sleep, usually feels less groggy.
- Why does it add 15 minutes?
- Most people take around 10 to 20 minutes to actually fall asleep after getting into bed. The calculator adds 15 minutes so the cycle count starts from when you are likely asleep, not when your head hits the pillow.
- How many cycles should I aim for?
- Five to six cycles, roughly 7.5 to 9 hours, suits most adults. Four cycles (about 6 hours) is a short night that can work occasionally but is not ideal as a routine.