Find the best time to go to bed or wake up so you finish a full 90 minute sleep cycle and feel less groggy.
How to use
- Choose whether you know your wake up time or your bedtime.
- Pick the time.
- Press Calculate and choose one of the suggested times. The tool allows 15 minutes to fall asleep.
Good to know
A sleep cycle lasts about 90 minutes on average, but yours may differ a little. Explore more on the Tools page.
Why sleep cycles matter
Sleep is not one long block. During the night your brain moves through repeating stages, from light sleep to deep sleep and then REM sleep, which is when most vivid dreams happen. One full trip through these stages is called a sleep cycle and lasts about 90 minutes for most adults. If your alarm goes off in the middle of deep sleep you tend to wake up heavy and confused, a feeling often called sleep inertia. If it goes off at the end of a cycle, when your sleep is lighter, getting up usually feels easier.
That is the idea behind this sleep calculator. Tell it when you need to wake up, and it counts backward in 90 minute steps to suggest bedtimes. Or tell it when you plan to go to bed, and it counts forward to suggest wake up times. It also adds fifteen minutes, which is a typical time for a healthy person to fall asleep.
How the calculator works
For a fixed wake up time, the tool subtracts the time you need to fall asleep and then subtracts whole cycles of 90 minutes. Six cycles equal nine hours, five cycles equal seven and a half hours, four cycles equal six hours and three cycles equal four and a half hours. For a fixed bedtime it works the other way around by adding fifteen minutes and then whole cycles. Every result is shown in a twelve hour clock with AM and PM, and the options with five or six cycles are marked as recommended for most adults.
How much sleep do you need?
General health guidance says most adults do best with seven to nine hours of sleep a night. Teenagers need more, often eight to ten hours, and young children need even more. These are broad ranges, and individual needs vary. If you wake refreshed and stay alert all day with seven and a half hours, you are probably getting what you need. If you feel tired even after nine hours, consider talking to a doctor, because ongoing tiredness can have causes such as sleep apnea, stress, low iron or thyroid problems.
Example
Say you must be up at 6:30 AM. Counting back, the calculator suggests going to sleep around 9:15 PM for six cycles, 10:45 PM for five cycles, 12:15 AM for four cycles and 1:45 AM for three cycles. Because you also need about fifteen minutes to doze off, the times above already include that buffer. You would aim to be in bed with the lights out at the suggested time, not just start brushing your teeth then.
Tips for better sleep
- Keep a regular schedule. Going to bed and waking up at the same time every day, including weekends, trains your body clock.
- Make your room dark, cool and quiet. A temperature around 18 to 20 degrees Celsius suits many people.
- Stop using screens at least thirty to sixty minutes before bed. Bright light, and engaging content, keeps your brain alert.
- Limit caffeine in the afternoon. It can stay in your body for many hours.
- Avoid heavy meals and alcohol close to bedtime. Alcohol may make you drowsy but it disrupts sleep later in the night.
- Get daylight and some exercise during the day, but not intense workouts right before bed.
Is a 90 minute cycle exact?
No. The 90 minute figure is an average. Real cycles range from about 80 to 110 minutes, and the length can change through the night, with more deep sleep earlier and more REM sleep later. Think of the results as helpful targets rather than exact science. If you try a suggested time for a week and still feel tired, shift your bedtime by fifteen minutes in either direction and see how you feel.
Naps
The same idea applies to naps. A short nap of 15 to 20 minutes keeps you in light sleep and usually leaves you refreshed. A nap of 90 minutes covers a full cycle. Naps that last between 30 and 60 minutes often end in deep sleep, which is why they can leave you groggy. Try to nap before mid afternoon so that you can still fall asleep at night.
Related tools
If you like tracking your health habits, you may also want our BMI Calculator, Age Calculator and Date Difference Calculator. This tool is for general wellbeing and is not medical advice.
Frequently asked questions
How many sleep cycles do I need?
Five or six cycles, which is 7.5 to 9 hours, suits most adults.
How long does it take to fall asleep?
About 10 to 20 minutes for many people, so the calculator uses 15.
Can I use it for naps?
Yes. Use the idea of a short 20 minute nap or a full 90 minute nap.
Does it replace medical advice?
No. If you have long lasting trouble sleeping, see a doctor.
Is my data saved?
No. Everything runs in your browser.
Signs you may need more sleep
Many people think they are used to short nights, but certain signs suggest a sleep debt. You may need an alarm and several snoozes every morning, feel sleepy in meetings or while reading, rely heavily on caffeine, become irritable, forget small things, or fall asleep within a minute of lying down. Falling asleep that quickly is not a sign of strength, it often means that you are very tired. If you recognise these signs, try moving bedtime earlier by fifteen minutes every few days until you wake up before the alarm.
Sleep stages in more detail
- Light sleep: the entry stage, when your heart rate slows and you can be woken easily.
- Deep sleep: the stage that restores the body, helps the immune system and is hardest to wake from.
- REM sleep: the stage with vivid dreams, linked with memory and learning.
Across the night you move through these stages several times. Deep sleep is more common early and REM becomes longer toward morning, which is one reason why cutting sleep short at the end can reduce REM.
A simple evening routine
- Pick a fixed wake time and count back with this calculator.
- Dim the lights an hour before bed.
- Put the phone away or use night mode.
- Write down tomorrow tasks so your mind can rest.
- Read, stretch or listen to calm music.
When to see a doctor
If you snore loudly, stop breathing in your sleep, wake up gasping, cannot fall asleep for weeks or feel exhausted despite enough hours, speak to a health professional. Sleep problems are common and often treatable.