What are sleep cycles?
Sleep is not uniform. Over the night, several stages follow one another repeatedly: light sleep, deep sleep and REM, the stage in which we most often dream. One such round lasts about 90 minutes on average, and an adult sleeps through four to six of them a night.
If an alarm wakes you in the middle of deep sleep, you may feel groggy and drowsy, which experts call sleep inertia. At the end of a cycle sleep is lighter, so getting up is often easier. That is why the calculator counts in steps of 90 minutes and adds about 15 minutes, which is how long most people need to fall asleep.
How much sleep do I need?
The American Academy of Sleep Medicine (AASM) and the National Sleep Foundation recommend 7 to 9 hours of sleep a night for adults and 7 to 8 hours for people over 65. Teenagers need 8 to 10 hours, school-age children 9 to 12 hours.
For most adults, five or six cycles (7.5 or 9 hours) therefore make the most sense. Four cycles (6 hours) are fine for a single night, but if you sleep like that for a long time, a sleep debt builds up.
| Age | Hours per night |
|---|---|
| 6–12 years | 9–12 hours |
| 13–18 years | 8–10 hours |
| 18–64 years | 7–9 hours |
| 65 years or more | 7–8 hours |
Habits for a better evening
Among good habits, guidelines list a regular schedule – a similar bedtime and wake-up time every day, weekends included –, a dark, quiet and cooler bedroom, fewer screens in the last hour before bed, and caffeine only until early afternoon.
If you regularly sleep badly despite enough time in bed, wake often during the night, snore loudly with pauses in breathing or are constantly sleepy during the day, talk to your doctor. The calculator is a planning tool, not an assessment of the quality of your sleep.
Frequently asked questions
When should I go to bed if I get up at 6:00?
For five cycles (7.5 hours) plus 15 minutes to fall asleep, go to bed at 22:15; for six cycles (9 hours), at 20:45. For four cycles (6 hours), bedtime is 23:45.
How long is one sleep cycle?
About 90 minutes on average, and from roughly 80 to 110 minutes for an individual. Towards morning, cycles are often longer and contain more REM sleep.
Is 7.5 hours of sleep better than 8 hours?
Not necessarily. 7.5 hours lines up with five average cycles, but since cycle length varies, it matters more that you regularly sleep 7 to 9 hours and get up at a similar time.
How long does it take me to fall asleep?
Most healthy adults fall asleep within 10 to 20 minutes. The calculator therefore allows 15 minutes. If you regularly need more than 30 minutes, talk to your doctor about it.
Is a daytime nap okay?
A short nap of around 20 minutes is fine for most people. Longer naps, or a nap late in the afternoon, can push back the time you fall asleep in the evening.
Sources
- Watson NF et al. – Recommended Amount of Sleep for a Healthy Adult: AASM and SRS (2015)
- Hirshkowitz M et al. – National Sleep Foundation’s sleep time duration recommendations (2015)
- NHS – How to get to sleep
- NIH NINDS – Brain Basics: Understanding Sleep
Updated: October 2026
For information only, not a substitute for medical advice.