Health / 07
Sleep Calculator
Find the best time to go to bed for the time you need to wake up, or when to wake if you go to bed now, counting whole 90-minute sleep cycles and the time it takes to fall asleep.
Sleep Calculator: Sleep runs in cycles of light, deep, and dream (REM) sleep that average about 90 minutes. Waking at the end of a cycle, in light sleep, usually feels easier than waking from deep sleep. The calculator counts back or forward in whole cycles from the time you give, adding the minutes you take to fall asleep, and lists 3 to 6 cycles. Runs 100% locally in your browser with zero server file uploads.
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- Health & sport tools
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- In your browser
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- Free · no sign-up
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- Ready to use
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Go to bed at 11:15 PM
5 cycles, 7.5 hours of sleep| Go to bed | Sleep cycles | Sleep |
|---|---|---|
| 9:45 PM | 6 | 9 hours |
| 11:15 PM | 5 | 7.5 hours |
| 12:45 AM | 4 | 6 hours · short night |
| 2:15 AM | 3 | 4.5 hours · short night |
Adults need at least 7 hours of sleep a night, so 5 or 6 cycles suit most people. A cycle averages about 90 minutes but varies from person to person, so treat these as starting points and adjust by 10 to 15 minutes if you still wake up groggy. Set an alarm
Inside a night's sleep
Each cycle moves from light sleep into deep sleep and then into REM sleep, when most dreaming happens. Early cycles hold the most deep sleep, which the body uses for physical recovery; later ones hold more REM sleep. Cutting a night short mostly costs REM sleep, which is why the last hour or two matter more than they seem.
Waking during deep sleep can leave you groggy for half an hour or more, which sleep researchers call sleep inertia. Waking near the end of a cycle, in lighter sleep, usually feels easier.
Making the times stick
A fixed wake-up time, weekends included, sets the body clock more firmly than a fixed bedtime. Set it with the alarm clock; if noise keeps you awake, steady background sound from the white noise generator can mask it. Daylight soon after waking helps too.
How to use it
- Choose whether you know your wake-up time, your bedtime, or are going to bed now.
- Enter the time, and how long you usually take to fall asleep.
- Pick a bedtime or wake-up time from the list; 5 or 6 cycles suit most adults.
Privacy & limitations
Everything is calculated in your browser. Your usual wake-up time is kept only in this browser, for next time.
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Frequently asked questions
How much sleep do adults need?
The American Academy of Sleep Medicine and the Sleep Research Society recommend at least 7 hours a night for adults. Five cycles give 7.5 hours of sleep and six give 9; four cycles, 6 hours, is short for most people if it happens often.
Is a sleep cycle always 90 minutes?
No. It averages around 90 minutes but ranges from about 70 to 120, differs between people, and changes through the night, with more deep sleep early on and more dream sleep towards morning. Use the times as a starting point and shift them by 10 to 15 minutes if you still wake up groggy.
How long should it take to fall asleep?
Healthy adults usually fall asleep within 10 to 20 minutes. Falling asleep the moment your head touches the pillow can be a sign of not getting enough sleep; regularly lying awake for more than half an hour is worth mentioning to a doctor.
Is it better to sleep less but wake at the end of a cycle?
Getting enough sleep matters more than the timing of the alarm. If the choice is between 6 hours timed to a cycle and 7 hours that is not, the extra hour usually helps more.
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