How sleep cycles work
During the night you pass through several sleep cycles in a row. Each cycle consists of light sleep, deep sleep and REM sleep (the stage in which you dream most). A cycle lasts about 90 minutes on average, although it varies between people and from night to night, roughly between 70 and 120 minutes. An adult typically goes through four to six cycles a night.
The idea behind this calculator is simple: if you wake up at the end of a cycle, when sleep is lighter, you often feel fresher than when your alarm drags you out of deep sleep. That groggy feeling after waking is called sleep inertia.
How the calculator works
- Waking up at a set time: the calculator counts back in blocks of 90 minutes and subtracts the time you need to fall asleep (15 minutes by default).
- Going to bed now or later: it adds the time to fall asleep to your bedtime, then adds blocks of 90 minutes.
You get options for 6, 5, 4 and 3 cycles. Five or six cycles, or 7.5 to 9 hours of sleep, fit best with the advice for adults.
Example
You want to wake up at 7:00 and usually need fifteen minutes to fall asleep.
- For 6 cycles (9 hours of sleep), go to bed at 21:45 (9:45 pm).
- For 5 cycles (7.5 hours), go to bed at 23:15 (11:15 pm).
- If that's not possible, 00:45 still gives 4 cycles (6 hours), which is too little for most adults over the long term.
If you go to bed at 23:00 (11 pm), 6:45 (5 cycles) and 8:15 (6 cycles) are good times to wake up.
How much sleep do you need?
Most health organisations recommend 7 to 9 hours of sleep a night for adults. Teenagers need more, about 8 to 10 hours, and young children more still. Older adults often sleep shorter and lighter, but don't necessarily need less sleep. You can tell how much you need by how you feel during the day: if you sleep much longer without an alarm or get sleepy easily during the day, you are probably not getting enough.
Tips for a better sleep routine
- Consistent times: go to bed and get up at roughly the same time every day, including weekends. Your body clock likes regularity.
- Light: get daylight in the morning and dim the lights in the evening. Bright screens right before bed can delay falling asleep.
- Caffeine and alcohol: coffee, tea and energy drinks keep working for hours. Alcohol may help you fall asleep, but it makes sleep more restless later in the night.
- Bedroom: a cool, dark and quiet room helps. Many people find around 16–18 °C (60–65 °F) comfortable.
- Exercise: regular activity improves sleep, but a hard workout right before bed can be stimulating.
A guide, not a rule
The 90 minutes is an average. Early cycles contain more deep sleep, REM sleep increases later in the night, and cycle length differs between people. Use the result as a starting point and experiment for a week or two with fifteen minutes earlier or later. If you have long-term sleep problems, such as trouble falling asleep, waking often, snoring with pauses in breathing or extreme daytime tiredness, talk to your doctor.
Frequently asked questions
What time should I go to bed if I wake up at 7?
At 21:45 (9:45 pm) for six cycles or 23:15 (11:15 pm) for five, assuming fifteen minutes to fall asleep.
How long is a sleep cycle?
About 90 minutes on average, varying between roughly 70 and 120 minutes per person and per night.
Is 6 hours of sleep enough?
Not for most adults over the long term. The recommendation is 7 to 9 hours a night.
Why am I tired after sleeping a lot?
Being woken from deep sleep can leave you groggy. An irregular schedule or poor sleep quality can also play a part.
What if it takes me longer than 15 minutes to fall asleep?
Adjust the time to fall asleep in the calculator. If you regularly lie awake for more than half an hour, talk to your doctor.
Does this work for naps?
A short nap of 10 to 20 minutes keeps you out of deep sleep. A full-cycle nap of about 90 minutes also works but takes more time.
Last reviewed: 2026-10-06. Results are estimates for information only.