How the sleep calculator works
Choose what you want to work out. When should I wake up? starts from your bedtime: it fills in the current time, and Now resets it if you change it. When should I go to bed? starts from the time your alarm has to ring. The answers update as you type, in your device's own 12- or 24-hour clock format. If your ideal bedtime always feels out of reach, take the chronotype test as well: it shows whether your body clock runs early or late.
The calculation is simple and fully visible. We assume you need about 15 minutes to fall asleep and that each sleep cycle lasts about 90 minutes. In the first mode the calculator adds 15 minutes to your bedtime and then 4, 5 or 6 cycles: in bed at 23:00, asleep around 23:15, so you could wake at 05:15 (6 hours), 06:45 (7.5 hours) or 08:15 (9 hours). The second mode runs the same sum backwards from your wake-up time. If you know you fall asleep faster or slower, or that your cycles feel shorter or longer, change Time to fall asleep and Sleep cycle length. Every option is recalculated at once.
The bar in the result shows the same thing as a picture: thin lines for cycle boundaries, a darker zone for 7–9 hours, and markers for 4, 5 and 6 cycles. Options inside 7–9 hours get a ✓. Unlike a one-line answer, you can see at a glance how a 20-minute later bedtime moves every option, and whether the night you are planning is long enough.
Why 90 minutes, and why it is only an average
During the night your brain moves through light sleep, deep sleep and REM sleep, then starts again. In their classic overview of normal human sleep, Mary Carskadon and William Dement describe this NREM–REM cycle as lasting about 90 minutes, with the first REM period arriving 80–100 minutes after you fall asleep. That is where the 90-minute rule comes from.
It is not a fixed clock. Cycle length differs between people and from cycle to cycle within one night: deep sleep dominates the early cycles and REM the later ones. How long you take to fall asleep also changes with stress, caffeine and how tired you are. So treat the times as good targets, not exact predictions. Waking at the end of a cycle, when sleep is lighter, may help you feel less groggy, but no calculator can know where your cycles really fall without measuring your sleep.
How much sleep you need
Total sleep matters more than hitting a cycle boundary. In 2015 the American Academy of Sleep Medicine (AASM) and the Sleep Research Society agreed that adults should sleep 7 or more hours a night on a regular basis. The same year a National Sleep Foundation panel recommended 7–9 hours for adults aged 18–64 and 7–8 hours for those 65 and over. That is why the ✓ marks the 7–9 hour options: with 90-minute cycles these are 5 cycles (7.5 hours) and 6 cycles (9 hours). Four cycles (6 hours) is shown so you can see the trade-off on a short night, but it falls below the recommendation.
What you can do with the result
- Fix your wake-up time first. A steady wake time, weekends included, anchors your body clock. Use the bedtime mode to work backwards from it.
- Notice your own pattern. If you wake up naturally a few minutes before the alarm on most days, your cycles probably fit the times you chose. If not, try a 10-minute shift.
- Check your chronotype. If the "right" bedtime always feels impossible, your body clock may run late. The chronotype test linked above shows whether you are a morning or evening type.
- Do not save sleep for the weekend. Sleeping until noon on Saturday makes Monday morning feel like jet lag. After a short night, sleeping in by an hour at most and going to bed a little earlier is a better way to catch up.
- Go to bed when you feel sleepy. Lying awake watching the clock builds tension; use the times here to see which cycle you will land in once sleep comes.
- See how sleep affects you. After a short night, try the reaction time test or the attention span test and compare your scores with a well-rested day.
Limits: not a sleep diagnosis
This calculator uses averages and your own settings. It does not measure your sleep, and it cannot tell whether you have a sleep problem. See a doctor if you regularly lie awake for more than 30 minutes, still feel exhausted after 7–9 hours in bed, snore loudly or someone notices pauses in your breathing, or fall asleep during the day without meaning to. These can be signs of insomnia, sleep apnoea or other conditions that need proper assessment.
More tools for your daily rhythm are collected on our sleep tests page.
Sources
- Carskadon, M. A., & Dement, W. C. (2011). Normal human sleep: An overview. In M. H. Kryger, T. Roth & W. C. Dement (Eds.), Principles and Practice of Sleep Medicine (5th ed., pp. 16–26). Elsevier Saunders.
- Watson, N. F., Badr, M. S., Belenky, G., 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.
- Hirshkowitz, M., Whiton, K., Albert, S. M., et al. (2015). National Sleep Foundation's sleep time duration recommendations: Methodology and results summary. Sleep Health, 1(1), 40–43.
Frequently asked questions
How does a sleep calculator work?
It adds the time you need to fall asleep (15 minutes by default) to your bedtime and then whole sleep cycles of about 90 minutes. The results are wake-up times at the end of 4, 5 or 6 cycles. In bedtime mode it does the same sum backwards from your alarm.
How long is one sleep cycle?
About 90 minutes on average, according to the classic description by Carskadon and Dement. Real cycles vary between people and through the night, so you can set the cycle length to 80, 90 or 100 minutes.
How many hours of sleep do adults need?
The AASM and Sleep Research Society recommend 7 or more hours a night for adults. The National Sleep Foundation recommends 7–9 hours for adults aged 18–64 and 7–8 hours from age 65.
Is 6 hours of sleep enough if it is 4 full cycles?
Finishing a cycle may make waking easier, but 6 hours is still below the 7-hour minimum recommended for adults. Use it for an occasional short night, not as a routine.
What if I am in bed now?
Choose When should I wake up? and press Now. The calculator uses the current time, adds 15 minutes to fall asleep and shows wake-up times for 4, 5 and 6 cycles.
Why are the times shown in 12-hour (or 24-hour) format?
The calculator follows your device's own clock setting, so you see times the way you are used to reading them.
Does this calculator work for children?
The cycle maths is the same, but children and teens need more sleep than adults and their cycles are shorter. The 7–9 hour marks in this tool are adult recommendations.