Sleep Calculator
Sleep Calculator
Find the best time to go to bed for the time you need to wake up, or the best time to wake up if you are going to bed now, by counting whole sleep cycles of about 90 minutes plus the time it takes to fall asleep, checked against the hours recommended for your age.
Your night in sleep cycles
Hours of sleep by option
Sleep runs in cycles through light, deep and REM sleep, and the National Institute of Neurological Disorders and Stroke notes that REM sleep first occurs about 90 minutes after falling asleep. Cycles vary from person to person and through the night, so 90 minutes is an average; change it if you know your own rhythm. Waking at the end of a cycle, in lighter sleep, usually feels easier than waking from deep sleep. Recommended hours are from the CDC, based on the American Academy of Sleep Medicine and the National Sleep Foundation. This is a planning aid, not medical advice; persistent trouble sleeping is worth raising with a doctor. The CSV, image and printed report contain the values you entered, nothing preloaded.
How to use this tool
Choose whether you know the time you need to wake up or the time you are going to bed, and enter it. Pick your age group, and adjust how long you usually take to fall asleep and the length of your sleep cycles if you know them. The best time appears at the top, with every other option below it.
The timeline shows the night split into cycles, and the bar chart compares the hours of sleep each option gives with the range the CDC recommends for your age.
How the times are worked out
Through the night you move through light sleep, deep sleep and REM sleep, then start again. Each round is a sleep cycle, about 90 minutes on average. You tend to wake more easily near the end of a cycle, when sleep is lighter, and feel groggier if an alarm drags you out of deep sleep.
So the calculator counts back from your wake up time in whole cycles, then subtracts the time it takes you to fall asleep. Going the other way, it adds the time to fall asleep to your bedtime and counts forward. The best option is the fewest cycles that give the hours recommended for your age.
Bedtimes for common wake up times
| Wake up at | 6 cycles, 9 h | 5 cycles, 7 h 30 min | 4 cycles, 6 h |
|---|---|---|---|
| 5:00 AM | 7:45 PM | 9:15 PM | 10:45 PM |
| 5:30 AM | 8:15 PM | 9:45 PM | 11:15 PM |
| 6:00 AM | 8:45 PM | 10:15 PM | 11:45 PM |
| 6:30 AM | 9:15 PM | 10:45 PM | 12:15 AM |
| 7:00 AM | 9:45 PM | 11:15 PM | 12:45 AM |
| 7:30 AM | 10:15 PM | 11:45 PM | 1:15 AM |
| 8:00 AM | 10:45 PM | 12:15 AM | 1:45 AM |
| 8:30 AM | 11:15 PM | 12:45 AM | 2:15 AM |
| 9:00 AM | 11:45 PM | 1:15 AM | 2:45 AM |
Bedtimes allowing 15 minutes to fall asleep and 90 minute cycles. For adults 18 to 60, 5 or 6 cycles meet the CDC's 7 or more hours.
A worked example you can check on paper
You need to be up at 7:00 AM. Five cycles of 90 minutes is 450 minutes, 7 hours 30 minutes, so you need to be asleep by 11:30 PM. Allowing 15 minutes to drift off, go to bed at 11:15 PM. Six cycles instead would mean 9 hours asleep and a 9:45 PM bedtime; four would be only 6 hours, below the CDC's 7 hour minimum for adults.
Mistakes that catch people out
Counting time in bed as sleep. The minutes spent falling asleep, and any waking in the night, do not count toward your cycles.
Treating 90 minutes as exact. Cycles differ between people and lengthen toward morning. If an alarm keeps catching you in deep sleep, shift it by 10 to 20 minutes.
Sleeping in at weekends. A late weekend lie in shifts your body clock and makes Monday harder. A fixed wake up time helps most.
Cutting to four cycles regularly. Six hours might feel fine on one morning, but it is short of what adults need night after night.
Popular calculations
Each opens the calculator with its own address, and every value can be changed once it is open.
Bedtime for your wake up time
Wake up time for your bedtime
Teens, children and older adults
Other cycle lengths and sleep onset
How this was checked
The engine was tested in Node against hand calculations, including times that cross midnight: a 7:00 AM wake up gives bedtimes of 11:15 PM and 9:45 PM, a 6:30 AM wake up for a teen gives 9:15 PM, and an 11:00 PM bedtime gives wake up times of 6:45 AM and 8:15 AM. Recommended hours were checked against the CDC table.
Every address in the lists above was then opened and checked for a distinct title, the right answer, charts that render, and clean CSV, image and print output containing only that calculation, at desktop, tablet and phone widths.
Common questions
What time should I go to bed if I wake up at 6 AM?
Allowing 15 minutes to fall asleep and 90 minute cycles, go to bed at 10:15 PM for five cycles, 7.5 hours of sleep, or 8:45 PM for six cycles, 9 hours. Both meet the 7 or more hours the CDC recommends for adults.
How long is a sleep cycle?
About 90 minutes on average, though it varies from person to person and through the night, commonly between about 70 and 120 minutes. The National Institute of Neurological Disorders and Stroke notes that REM sleep first occurs about 90 minutes after falling asleep.
How many sleep cycles do I need?
Five cycles of 90 minutes is 7.5 hours and six is 9 hours, which covers the 7 or more hours the CDC recommends for adults. Teens need 8 to 10 hours and school age children 9 to 12.
Is it better to sleep 6 hours or 7.5 hours?
7.5 hours. Waking at the end of a cycle can make 6 hours feel better than 6.5, but 6 hours is below the 7 hours the CDC recommends for adults, and regularly sleeping less than that is linked with health problems.
How long does it take to fall asleep?
Most people take roughly 10 to 20 minutes, and the calculator uses 15 by default. If you regularly take much longer, or fall asleep the instant your head hits the pillow, that is worth mentioning to a doctor.
Why do I wake up tired after 8 hours?
Waking in the middle of deep sleep can leave you groggy for a while, which is what cycle counting tries to avoid. Irregular sleep times, alcohol, caffeine late in the day and sleep disorders such as sleep apnea are other common causes.
Related Calculators and Reference
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- Sunrise and Sunset Calculator: sunrise time and day length at your location, useful for setting a wake time by daylight.
- Caffeine Half Life Calculator: how much caffeine is still in your body at bedtime.
- CDC: About Sleep: recommended hours of sleep by age.