Free sleep cycle calculator · no sign-up
Sleep calculator: plan your night in sleep cycles
Late finish tonight, early start tomorrow? This sleep calculator counts in sleep cycles of about 90 minutes. Enter the time you need to get up, or the time you plan to go to bed. You get three times to choose from and a short light plan to go with them.
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- Whole 90-minute cycles
- Light plan included
- Your times are not saved or sent
The method
How your times are worked out
Through the night you move between sleep stages: light sleep, slow-wave sleep and REM (dream) sleep. One round of these stages is called a sleep cycle, and an average one lasts about 90 minutes. Waking up during slow-wave sleep tends to leave you groggy for longer.
The maths is simple: your wake-up time, minus 6, 5 or 4 cycles of 90 minutes, minus the minutes you need to fall asleep, gives a bedtime. Start from a bedtime and it adds the same minutes and cycles to find an alarm time. Nothing here measures your own sleep; the times come from averages. Getting enough sleep matters more than hitting a cycle exactly: most adults need 7 to 9 hours. Five cycles (7.5 hours) and six (9 hours) fall in that range, so they are marked recommended. For people aged 65 or over the advice is 7 to 8 hours, so only the five-cycle option fits. Four cycles, or 6 hours, is shown as a short night.
The light plan follows recommendations on indoor light from an international group of sleep and light researchers (Brown and colleagues, PLoS Biology, 2022). In short: bright light by day, dim light in the three hours until lights out, and darkness while you sleep. The plan's times are set around the 5-cycle option of 7.5 hours.
Where the figures come from
Common questions
How long is a typical sleep cycle?
On average about 90 minutes, within a range of roughly 70 to 120 minutes. The length differs between people and from one cycle to the next. Slow-wave sleep is more common early in the night, and REM (dream) sleep towards morning.
How much sleep do I need?
The US National Sleep Foundation advises 7 to 9 hours for anyone aged 18 to 64. From 65 the range drops to 7 to 8 hours, and for teenagers it is 8 to 10 hours. Your best guide is how alert and rested you feel in the daytime.
Why do I wake up groggy when my alarm goes off?
If the alarm goes off during slow-wave sleep, it takes longer before you feel properly awake. That groggy feeling after waking is called sleep inertia. Cycle lengths vary, so counting in 90-minute steps can only get you near the end of a cycle. Many people find mornings easier with the same wake-up time on workdays and days off, and daylight in the first hour after you wake up.
What has light got to do with when I sleep?
Your body clock uses light to tell day from night. A bright morning and a well-lit day set it to daytime. A dim evening and a dark bedroom set it to night. In a city that stays lit late, the dark part takes some planning.
Does the calculator save my times?
No. Everything is worked out in your browser, and your times are not saved or sent anywhere.
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