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CalculatorLab

Sleep Calculator

Tell us when you need to wake up, or when you are going to bed, and we will suggest times that fit whole sleep cycles.

I want to

We suggest bedtimes that end a sleep cycle at that time.

Best time to go to bed (5 cycles, 7.5 hours)

11:15 PM

Sleep cycle
90 minutes
Time to fall asleep
15 min
9:45 PM6 cycles · 9 h
11:15 PM5 cycles · 7.5 h
12:45 AM4 cycles · 6 h
2:15 AM3 cycles · 4.5 h

Most adults need 7 to 9 hours of sleep. A cycle is only an average of about 90 minutes and varies from person to person and through the night. This is a planning aid, not medical advice: if you often struggle to sleep or feel tired all day, talk to a doctor.

What a sleep cycle is

Sleep is not one long, even state. Through the night you move through stages: light sleep, deep sleep and REM sleep, in cycles of roughly 90 minutes. Early cycles contain more deep sleep, and later ones more REM sleep and lighter sleep.

An alarm that goes off in deep sleep can leave you groggy, an effect called sleep inertia. An alarm near the end of a cycle, when sleep is lighter, is often easier to get up to. The idea of this calculator is to line up your wake time with the end of a cycle.

A worked example

You need to be up at 7:00 and take about 15 minutes to fall asleep. Six cycles is 9 hours, so the bedtime is 9:45 PM. Five cycles is 7.5 hours, so the bedtime is 11:15 PM. Four cycles is 6 hours, so it is 12:45 AM, and three cycles is 4.5 hours, so it is 2:15 AM. If you go to bed at 11:00 PM instead, the wake-up times are 3:45, 5:15, 6:45 and 8:15 AM.

Tips for better sleep

A regular schedule helps most: go to bed and wake up at about the same times every day, including weekends. Keep the bedroom dark, quiet and cool, avoid caffeine in the afternoon, and put screens away for the last half hour.

This tool is not a medical device and cannot diagnose sleep problems. If you snore loudly, stop breathing at night or have long-lasting trouble sleeping, see a doctor.

Questions people ask

How does the sleep calculator work?

Sleep moves through repeating cycles that last about 90 minutes on average, from light sleep to deep sleep and REM. Waking at the end of a cycle, rather than in the middle of deep sleep, often feels easier. We count back (or forward) in 90-minute steps from the time you give us, adding the time it takes you to fall asleep.

How many hours of sleep do I need?

Most adults need 7 to 9 hours a night, which is 5 or 6 cycles. Teenagers and children need more, and some older adults sleep a little less. If you wake up tired after a full night, or fall asleep during the day, speak to a doctor.

Why add time to fall asleep?

Nobody falls asleep the instant the light goes out. The average is about 10 to 20 minutes. We add the minutes you enter (15 by default) so your cycles start when you are really asleep. If you fall asleep faster or slower, change the number.

Is a 90-minute cycle exact?

No. Cycle length averages about 90 minutes but ranges from roughly 70 to 110 minutes between people, and it changes through the night. Treat the times as a good starting point and adjust if you wake up groggy.

What if I can only get 3 or 4 cycles?

Short nights are sometimes unavoidable, so we show 3, 4, 5 and 6 cycles. Four and a half to six hours is a short night, though, and over time too little sleep affects mood, memory and health. Aim for 5 or 6 cycles whenever you can.