Sleep Calculator
Enter the time you need to wake up and get a bedtime that fits whole sleep cycles, or enter your bedtime and get matching wake-up times. Age group, time to fall asleep and cycle length are adjustable.
Created by the Reeliy Editorial Team. Last updated: Oct 19, 2026
Using the sleep calculator
- Choose what to work out: a bedtime for a wake-up time you must hit, or a wake-up time for a bedtime you already have in mind.
- Enter the time. The default is 7:00 AM for waking up and 11:00 PM for going to bed.
- Pick the age group, since sleep needs and cycle length change from infancy to old age. Choosing a group fills in typical values for the two boxes below.
- Adjust the time you take to fall asleep and the length of your sleep cycle if you know them. The defaults (15 and 90 minutes) suit most adults.
The results show two options. The recommended time covers the number of full sleep cycles typical for your age group, and the minimum sleep time is one cycle shorter, for nights when the full amount is out of reach. Each option lists how long you would sleep, roughly how much of that is REM sleep, and how many cycles it contains.
How the times are worked out
Sleep is built from repeating cycles. The calculator counts back (or forward) a whole number of cycles and allows for the time it takes to drift off:
bedtime = wake-up time − (cycles × cycle length) − time to fall asleep
wake-up time = bedtime + time to fall asleep + (cycles × cycle length)
Example, finding a bedtime: you must be up at 6:30 AM, you are an adult with 90-minute cycles and you take 15 minutes to fall asleep. Six cycles are 9 hours, so the recommended bedtime is 6:30 − 9:00 − 0:15 = 9:15 PM. Five cycles are 7.5 hours, giving a latest bedtime of 10:45 PM.
Example, finding a wake-up time: you get into bed at 11:00 PM. Add 15 minutes to fall asleep, then six cycles (9 hours) to reach 8:15 AM, or five cycles (7.5 hours) to reach 6:45 AM. Four cycles would end at 5:15 AM.
The REM figure is a share of the total sleep. For adults it is 20 percent, so 9 hours (540 minutes) contains about 108 minutes, or 1 hour 48 minutes, of REM sleep.
What a sleep cycle is
During the night the brain moves through light sleep, deep sleep (also called slow-wave sleep) and REM sleep, then starts again. One pass takes about 90 minutes in a typical adult, although it varies from person to person and from night to night, commonly between about 80 and 120 minutes. Deep sleep dominates the early cycles of the night, while REM periods lengthen towards morning.
The idea behind counting in cycles is that waking at the end of one, when sleep is light, tends to feel easier than being pulled out of deep sleep, which often causes grogginess known as sleep inertia. Treat the 90-minute figure as an average rather than a precise rule. If you reliably wake up foggy at the suggested time, try shifting it by 10 or 15 minutes, or change the cycle length setting.
How much sleep each age group is given
Babies and young children have shorter cycles and spend far more of the night in REM sleep, so the calculator uses fixed cycle lengths and REM shares for them. These are the values it works with for one night of sleep (naps are not included):
| Age group | Cycle length | Recommended | Minimum | REM share |
|---|---|---|---|---|
| 0-3 months | 40 min | 16 cycles (10 h 40 min) | 14 cycles (9 h 20 min) | 50% |
| 4-11 months | 50 min | 13 cycles (10 h 50 min) | 12 cycles (10 h) | 40% |
| 1-2 years | 60 min | 11 cycles (11 h) | 10 cycles (10 h) | 30% |
| 3-4 years | 70 min | 9 cycles (10 h 30 min) | 8 cycles (9 h 20 min) | 26% |
| 5-13 years | 90 min | 7 cycles (10 h 30 min) | 6 cycles (9 h) | 23% |
| 14-17 years | 90 min | 7 cycles (10 h 30 min) | 6 cycles (9 h) | 22% |
| 18-24 years | 90 min | 6 cycles (9 h) | 5 cycles (7 h 30 min) | 22% |
| 25-64 years | 90 min | 6 cycles (9 h) | 5 cycles (7 h 30 min) | 20% |
| 65 and over | 90 min | 5 cycles (7 h 30 min) | 4 cycles (6 h) | 18% |
Published guidelines are expressed as ranges of hours. The US National Sleep Foundation's 2015 expert panel recommended 14 to 17 hours a day for newborns, 12 to 15 for infants, 11 to 14 for toddlers, 10 to 13 for preschoolers, 9 to 11 for school-age children, 8 to 10 for teenagers, 7 to 9 for adults up to 64 and 7 to 8 for people over 65. The figures in the table are planning targets for a single night, so they do not replace those ranges. The minimum column is the shortest night the calculator will plan for.
Time to fall asleep and cycle length
- Time to fall asleep (5 to 45 minutes): most healthy adults take 10 to 20 minutes. Whole minutes only. A value of exactly 5 is treated as the default 15, and for children under five the setting cannot drop below the typical time for their age group (30, 30, 20 and 15 minutes, from the youngest group up to 3-4 years).
- Sleep cycle duration (20 to 180 minutes): leave it at 90 unless you know your own pattern, for example from a sleep tracker. Picking an age group fills it in for you: 40, 50, 60 and 70 minutes for children from the youngest group up to 3-4 years, and 90 for everyone older. For children under five that value is always used, whatever is typed.
- Bedtime versus lights-out: enter when you intend to get into bed, and use the fall-asleep setting for the minutes you usually lie awake before drifting off. Time spent on a phone in bed belongs in that setting too.
Habits that help you keep to the plan
- Keep a regular schedule, including weekends, so your body clock settles on one rhythm.
- Get daylight in the morning and keep the evening dim, because light is the main signal for your body clock.
- Avoid caffeine in the afternoon and a heavy meal or alcohol close to bedtime. Alcohol may help you drop off but fragments sleep later in the night.
- Keep the bedroom cool, dark and quiet, and put screens away for the last stretch before bed.
- If you lie awake for a long time, get up and do something calm until you feel sleepy rather than watching the clock.
The times on this page are estimates built from averages, not medical advice. Long-lasting trouble sleeping, loud snoring with pauses in breathing, or heavy daytime sleepiness deserve a conversation with a doctor. See the disclaimer for more.
Related time tools
To add hours and minutes to any clock time, use the time calculator. To measure a span between two moments, try the date and time duration calculator, and to convert hours into minutes or seconds use the time converter.
Frequently asked questions
How much sleep does a person need each night?
Most adults do best on 7 to 9 hours a night, and a little less (7 to 8 hours) is typical after 65. Teenagers need 8 to 10 hours and school-age children 9 to 11 hours.
Why does the calculator use 90-minute cycles?
About 90 minutes is the average length of one cycle through light, deep and REM sleep in adults. Your own cycle may be shorter or longer, which is why the setting can be changed.
Is it better to sleep five cycles or six?
Six cycles (about 9 hours with the default settings) is the calculator's recommendation, while five (7.5 hours) is the shorter option. Many adults feel well on 7 to 9 hours, so either can work depending on how rested you feel.
Does the time it takes to fall asleep matter?
Yes. The cycles only start counting once you are asleep, so a 15-minute delay moves the bedtime 15 minutes earlier than the cycles alone would suggest.
References
- Hirshkowitz, M. et al. (2015). National Sleep Foundation's sleep time duration recommendations: methodology and results summary. Sleep Health, 1(1), 40-43.
- Watson, N. F. et al. (2015). Recommended amount of sleep for a healthy adult: a joint consensus statement of the American Academy of Sleep Medicine and Sleep Research Society. Sleep, 38(6), 843-844.
- Paruthi, S. et al. (2016). Recommended amount of sleep for pediatric populations: a consensus statement of the American Academy of Sleep Medicine. Journal of Clinical Sleep Medicine, 12(6), 785-786.
Cite this calculator and page
If you use this page in your work, you can cite it as:
Reeliy Editorial Team. (2026). Sleep Calculator. Reeliy. https://reeliy.com/calculators/sleep-calculator“Sleep Calculator.” Reeliy, 2026, https://reeliy.com/calculators/sleep-calculator.