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Sleep Calculator: Bedtimes and Wake-Up Times
Pick a wake-up time to see when to go to bed, or a bedtime to see when to set your alarm, using 90-minute sleep cycles and the time you usually take to fall asleep. A planning guide only.
How the cycle arithmetic works
During a night, sleep moves through repeated cycles of lighter sleep, deeper slow-wave sleep and rapid eye movement (REM) sleep. One full cycle is commonly described as lasting about 90 minutes, though in practice it ranges from roughly 80 to 110 minutes and changes across the night, with deeper sleep earlier and REM sleep longer toward morning. The idea behind this tool is that waking at the end of a cycle, in lighter sleep, may feel easier than being pulled out of deep sleep.
The calculation is plain subtraction or addition. To find a bedtime for a wake-up at 07:00 with 15 minutes to fall asleep and five cycles, it subtracts 15 minutes plus 5 × 90 = 450 minutes, giving 23:15. For a fixed bedtime it adds the same amounts forward. Times wrap around midnight.
How much sleep to aim for
The American Academy of Sleep Medicine and the Sleep Research Society issued a consensus statement in 2015 that adults should regularly sleep seven or more hours each night for good health. Needs vary, and teenagers, children and infants need more: school-age children commonly need 9 to 12 hours, and teenagers 8 to 10, according to the AASM guidance. The table lists what the cycle options mean for an adult:
| Cycles | Sleep | Comment |
|---|---|---|
| 6 | 9 hours | Long; fine if you need it |
| 5 | 7.5 hours | Within the adult guidance |
| 4 | 6 hours | Below the guidance for most adults |
| 3 | 4.5 hours | Occasional short night only |
Getting more from the times
- Be consistent. A steady wake time every day, including weekends, stabilises your body clock more than a perfect bedtime on one night.
- Set the fall-asleep time honestly. If you usually take 30 minutes, enter 30. Taking much longer than 30 minutes most nights is worth mentioning to a doctor.
- Allow for wind-down. Dim lights and screens off before the bed time shown.
- Watch caffeine and alcohol. Caffeine late in the day delays sleep and alcohol fragments it.
Limits
No tool can detect your actual sleep stages from a clock time. Sleep trackers differ in accuracy and a fixed 90-minute rule is a simplification, so how rested you feel matters more than hitting a cycle exactly. Persistent sleepiness, loud snoring with pauses in breathing, restless legs, insomnia lasting weeks, shift work or medicines that affect sleep deserve a conversation with a clinician. This page is educational and not medical advice.
Worked example
Someone who needs to be up at 06:30 usually takes about 15 minutes to fall asleep.
Inputs
| I want to | Wake up at a set time (find bedtimes) |
|---|---|
| Time (24-hour, HH:MM) | 06:30 |
| Minutes to fall asleep | 15 min |
| Cycle length | 90 min |
Result
To wake at 06:30 with 15 minutes to fall asleep and 90-minute cycles.
| 6 cycles (9.0 hours of sleep) | Be in bed at 21:15 (about 9 h) |
|---|---|
| 5 cycles (7.5 hours of sleep) | Be in bed at 22:45 (about 7.5 h) |
| 4 cycles (6.0 hours of sleep) | Be in bed at 00:15 |
| 3 cycles (4.5 hours of sleep) | Be in bed at 01:45 |
- Adults are generally advised to sleep seven or more hours a night, so the 5 and 6 cycle options are the ones to aim for; 3 and 4 cycles are a fallback for short nights.
- Cycle timing varies between people and through the night. Treat these times as a starting point and adjust by how you feel.
Medical disclaimer: This tool is for education and general planning. It is not medical advice and does not diagnose any condition. If you are pregnant, under 18, have an eating-disorder history or a medical condition, speak to a doctor or registered dietitian before changing your diet or exercise.
Frequently asked questions
How many sleep cycles do I need?
Most adults aim for five or six cycles, which is 7.5 to 9 hours of sleep at 90 minutes per cycle. Four cycles gives six hours, which is below the seven-hour guidance for most adults.
How long is a sleep cycle?
About 90 minutes on average, but it can range from roughly 80 to 110 minutes and varies from one person and one night to another. You can change the cycle length in the calculator.
Why should I add time to fall asleep?
Cycles start only when you are actually asleep. Most healthy adults take about 10 to 20 minutes to drop off, so the bedtime needs to include that gap to reach the planned amount of sleep.
Is waking at the end of a cycle really better?
Some people find it easier to wake in lighter sleep, but evidence for exact cycle timing is mixed. Total sleep and regular timing matter more than landing on a perfect multiple.
Does this work for naps or shift workers?
The same arithmetic applies to a daytime sleep, but your body clock may not support it. Short naps of 20 minutes or a full cycle are common choices, and shift workers should seek tailored advice.
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References and sources
- Watson NF, 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. Link
- Carskadon MA, Dement WC (2011). Normal human sleep: an overview. In Principles and Practice of Sleep Medicine, 5th edition. Elsevier.
- 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.