Sleep Calculator
Find when to go to bed, or when to wake up, so your alarm falls at the end of a sleep cycle.
About this tool
Sleep runs in cycles of about 90 minutes. Waking at the end of a cycle, rather than in the middle of deep sleep, usually feels easier. The calculator adds the time you take to fall asleep and suggests times for 3 to 6 full cycles, with 5 cycles (7.5 hours) highlighted.
Waking up in the middle of deep sleep can leave you groggy even after a full night, while waking at the end of a sleep cycle tends to feel easier. Tell the calculator when you need to get up, when you plan to go to bed, or that you are going to bed now, and it suggests times that line up with whole 90-minute cycles, allowing for the time it takes you to fall asleep.
How to use the Sleep Calculator
- Choose whether you have a set wake-up time, a set bedtime, or are going to bed now.
- Enter the time (not needed for going to bed now).
- Adjust how long you usually take to fall asleep; 15 minutes is typical.
- Pick one of the suggested times. Five or six cycles (7.5 or 9 hours) suits most adults.
Getting up at 6:30
Five 90-minute cycles is 7 hours 30 minutes. Counting back from 6:30 gives 23:00, and allowing 15 minutes to fall asleep means getting into bed at 22:45. Six cycles would mean bed at 21:15, four cycles at 00:15 and three cycles at 01:45. Going to bed at 23:00 instead, the calculator suggests alarms at 03:45, 05:15, 06:45 or 08:15.
Sleep cycles and how much sleep you need
Through the night you move through light sleep, deep sleep and REM (dreaming) sleep in cycles that average about 90 minutes, although they vary from roughly 70 to 120 minutes between people and through the night. Deep sleep is concentrated in the first cycles and REM in the later ones. Adults are generally advised to get 7 to 9 hours of sleep a night, teenagers 8 to 10 and school-age children 9 to 12. Cycle timing is a guide, not a rule: a regular bedtime and wake time, a dark and cool bedroom, and avoiding caffeine late in the day make a bigger difference. If you are often tired despite enough sleep, or you snore heavily, speak to a doctor.
Frequently asked questions
How long is a sleep cycle?
About 90 minutes on average, but anywhere from about 70 to 120 minutes depending on the person and the time of night.
How many hours of sleep do adults need?
Most adults need 7 to 9 hours. That is roughly five or six sleep cycles.
Why do I feel worse after a long nap?
A nap longer than about 30 minutes can reach deep sleep, and waking from it causes grogginess. Keep naps to 20 minutes or a full 90-minute cycle.
Is it bad to wake up in the middle of a cycle?
It is not harmful, but waking from deep sleep can leave you groggy for a while. This feeling, called sleep inertia, usually passes within 30 minutes.