Sleep calculator
Tell it when your alarm goes off and it counts back in sleep cycles of about 90 minutes, so you can aim to wake between cycles instead of in the middle of one. Or start from tonight’s bedtime and count forward.
Quick answer: To wake between sleep cycles, count back from your alarm in cycles of about 90 minutes after allowing time to fall asleep. For a 7:00 AM alarm and 15 minutes to drift off, aim for about 11:15 PM (five cycles, 7.5 hours of sleep) or 9:45 PM (six cycles, 9 hours). Real cycles vary from about 80 to 100 minutes.
Key facts
- Each sleep cycle lasts about 80 to 100 minutes, four to six a night (NHLBI)
- Adults need 7 to 9 hours (National Sleep Foundation panel)
- Five cycles of 90 minutes = 7.5 hours; six = 9 hours
- Runs in your browser; nothing you enter is stored or sent
Runs in your browser. Nothing you enter is stored or sent anywhere.
How it’s calculated
Bedtime = wake-up time − time to fall asleep − (number of cycles × cycle length). The calculator shows six, five, four and three cycles. Starting from a bedtime instead, wake-up time = bedtime + time to fall asleep + (cycles × cycle length), for three to six cycles.
- Cycle length: 90 minutes by default, with 80 and 100 minute options, from the NHLBI’s 80 to 100 minute range[1].
- Time to fall asleep: 15 minutes by default; you can choose 0 to 45 minutes.
- Highlighted results: options that give at least 7 hours (420 minutes) of sleep, the low end of the adult range[3].
Example: alarm at 7:00 AM, 15 minutes to fall asleep, 90-minute cycles. Five cycles is 450 minutes, so 7:00 AM − 15 min − 7 h 30 min = 11:15 PM.
How the sleep calculator works
Through the night you move through repeating cycles of light sleep, deep sleep and REM (dream) sleep. The National Heart, Lung, and Blood Institute describes each cycle as lasting about 80 to 100 minutes, with four to six cycles a night, and notes that you may wake briefly between them[1]. The NINDS adds that REM sleep first appears about 90 minutes after you fall asleep[2].
The calculator uses that pattern. It takes your alarm time, subtracts the time you usually need to drift off, then steps back in whole cycles of the length you choose (90 minutes by default). Each result is a bedtime that would let you finish a cycle right around the alarm. Bedtimes that give you 7 hours or more of sleep are highlighted, because that is the low end of what experts recommend for adults[2][3].
How to use the results
- Start with five or six cycles. Five cycles of 90 minutes is 7.5 hours of sleep and six is 9 hours. Three or four cycles fall short of the adult range of 7 to 9 hours[3], so treat them as an emergency option, not a plan.
- Be honest about the time to fall asleep. If you usually read for 20 minutes or lie awake a while, set that, or the bedtimes will be too late.
- Adjust after a week. Cycle length varies between people and from one night to the next[1], so no calculator can promise to wake you at the perfect moment. If you keep waking groggy, try the 80 or 100 minute setting, or simply a slightly earlier bedtime.
- Keep the wake-up time steady. A consistent schedule, including weekends, is easier to keep than a different bedtime every night. The NINDS notes that sleeping in on weekends may not fully make up for short nights[2]. Our weekly sleep debt calculator shows how short nights add up.
What it can and can’t tell you
The calculator is arithmetic built on an average. It can’t see your actual sleep stages, and the bedtime it suggests is a target for when to be in bed with the lights out, not a measurement. How much sleep you need also depends on your age: check the recommended hours for your age group before you settle on a number of cycles. If you want a short reset during the day instead, the nap timer works the same way for 10 to 90 minute naps.
Once you have a bedtime, a steady sound can help the house noise fade into the background. You can play brown noise, rain or pink noise in our free browser sound player, and our guide to playing sounds all night with the screen off covers the setup.
Questions
Is a sleep cycle always 90 minutes?
No. Ninety minutes is a convenient average. The NHLBI describes each cycle as lasting about 80 to 100 minutes, and the length can change during the night and from person to person. That is why the calculator lets you choose 80, 90 or 100 minutes.
What time should I go to bed to wake up at 6 AM?
With 15 minutes to fall asleep and 90-minute cycles, try 10:15 PM for five cycles (7.5 hours of sleep) or 8:45 PM for six cycles (9 hours). Enter your own wake-up time above to see every option.
How many sleep cycles do I need?
Most adults need 7 to 9 hours of sleep, which is roughly five to six cycles of 90 minutes. Children and teenagers need more. The right amount for you depends on your age and how you feel during the day.
Will waking between cycles stop me feeling groggy?
It may help, but it is not certain. Real cycles vary in length, so the calculator gives estimates. Getting enough total sleep on a regular schedule matters more than timing the alarm to the minute.
Does this sleep calculator store my times?
No. It runs entirely in your browser. Nothing you enter is sent anywhere or saved.
Sources
- National Heart, Lung, and Blood Institute. How Sleep Works: Sleep Phases and Stages. Updated March 24, 2022.
- National Institute of Neurological Disorders and Stroke. Brain Basics: Understanding Sleep. Reviewed February 25, 2025.
- Hirshkowitz M et al. National Sleep Foundation’s sleep time duration recommendations: methodology and results summary. Sleep Health, 2015.
This tool gives general estimates for healthy adults and is not medical advice. If you are often tired despite enough time in bed, snore loudly, or struggle to sleep on most nights, talk to a doctor or a sleep specialist.