😴 Sleep Calculator
Sleep runs in roughly 90-minute cycles, and waking mid-cycle is what causes grogginess. Enter your wake-up time and this calculator counts backwards to give six bedtimes — one per whole number of cycles.
Five or six full cycles (7.5 to 9 hours) suit most adults. The recommended bedtime below targets five cycles; adjust for the 15 minutes most people need to fall asleep.
How to use this calculator
Enter the time you must wake up. The calculator counts back in 90-minute sleep cycles and shows six ideal bedtimes, with the 5-cycle bedtime recommended.
- Enter your Wake-up time.
- Read the recommended bedtime for 5 full cycles.
- Browse the other five bedtimes for 1 to 6 cycles.
- Allow about 15 minutes to fall asleep before the listed time.
Frequently asked questions
What does the Sleep Calculator compute?
It takes your required wake-up time and subtracts multiples of 90 minutes to give six bedtimes, one per whole number of sleep cycles from 1 to 6. The 5-cycle bedtime, 7.5 hours of sleep, is highlighted as the recommendation.
Why 90-minute cycles?
Human sleep runs in roughly 90-minute cycles through light, deep and REM stages. Waking at the end of a cycle, rather than mid-cycle in deep sleep, is what makes mornings feel less groggy.
Can you give a worked example?
Waking at 6:30 AM gives bedtimes of 5:00 AM, 3:30 AM, 2:00 AM, 12:30 AM, 11:00 PM and 9:30 PM, with 11:00 PM recommended for 5 cycles.
Should I go to bed exactly at the listed time?
Treat it as lights-out minus the time you need to fall asleep. Most people need about 15 minutes, so aim to be in bed a quarter hour before the listed bedtime.