Sleep calculator

Free sleep calculator: find bedtimes or wake times that fit whole 90-minute sleep cycles, with the time it takes to fall asleep built in.

Calculate

Get into bed at one of these times to wake up at 7:00 AM:

  1. 9:45 PM6 cycles · 9 h7 h or more
  2. 11:15 PM5 cycles · 7 h 30 min7 h or more
  3. 12:45 AM4 cycles · 6 hUnder 7 h
  4. 2:15 AM3 cycles · 4 h 30 minLast resort

Includes 15 minutes to fall asleep and 90-minute cycles. Highlighted times give at least 7 hours of sleep.

How the sleep calculator works

Sleep runs in repeating cycles of light sleep, deep sleep and REM. In adults a cycle averages roughly 90 minutes, and the idea is to set your alarm near the end of one rather than partway through. The sleep calculator counts back from your alarm, or forward from your bedtime, in whole cycles and adds the minutes you need to fall asleep.

With the defaults, a 7:00 AM alarm works out like this: 6 cycles (9 hours) plus 15 minutes means getting into bed at 9:45 PM. Five cycles moves that to 11:15 PM and 4 cycles to 12:45 AM. The bedtime mode runs the same sum forward to give alarm times.

Times are worked out in real elapsed minutes, so on the night the clocks change for daylight saving the result already allows for the lost or extra hour.

Why 5 or 6 cycles are highlighted

In 2015 the American Academy of Sleep Medicine and the Sleep Research Society reviewed the evidence and agreed that adults aged 18 to 60 should sleep 7 or more hours a night on a regular basis.1 Regularly sleeping less was linked with weight gain, diabetes, high blood pressure, heart disease, stroke, depression and a higher risk of death.

At 90 minutes a cycle, 5 cycles is 7.5 hours and 6 is 9 hours, so those options carry the "7 h or more" mark. Change the cycle length and the highlight follows whichever options still clear 7 hours. The same statement says more than 9 hours may suit young adults, people recovering from sleep debt and people who are ill. For others it is uncertain whether that much sleep carries any risk.1

At the default 90 minutes, four cycles (6 hours) is listed but not highlighted. Three cycles is marked last resort: on a night with no other choice it may feel better than an alarm mid-cycle, but it is far short of 7 hours.

The 90-minute cycle is an average, not a rule

Cycle length varies a lot between people and between nights. A Basel lab study of 369 healthy adults, covering 6,064 recorded cycles, found a median of 96 minutes and a 95% range of 60 to 150 minutes.2 The first cycle of the night was consistently shorter than later ones. Age, sex and sleep deprivation all changed how long the NREM and REM parts of a cycle lasted.

So treat the results as a starting point. If you often wake groggy at a "cycle-aligned" time, move the cycle length up or down by 10 minutes for a week and see which alarm feels easier.

Grogginess after waking, called sleep inertia, does not come down to sleep stage alone. A review of the research found it is strongest when you wake during your biological night and stronger in recovery sleep after sleep deprivation, while the effect of waking from deep sleep varied by task and timing.3 A steady wake time and enough total sleep do more than landing on a cycle boundary to the minute.

What this bedtime calculator assumes

  • You fall asleep in 15 minutes. If you usually lie awake longer, change the number.
  • Every cycle is the same length. In reality the first is shorter.
  • You sleep straight through. Waking in the night shifts every cycle after it.
  • You are an adult. The 7-hour target comes from a statement on adults aged 18 to 60 and does not apply to children or teens.

If you can't fall asleep at the bedtime you want, the issue is usually body clock timing rather than cycle math. See how to fix your sleep schedule, check your caffeine cutoff, or use the nap calculator for daytime sleep.

Sources

  1. Watson NF, Badr MS, Belenky G, et al. Recommended amount of sleep for a healthy adult: a joint consensus statement of the American Academy of Sleep Medicine and Sleep Research Society. J Clin Sleep Med. 2015;11(6):591–592. doi:10.5664/jcsm.4758
  2. Cajochen C, Reichert CF, Münch M, et al. Ultradian sleep cycles: frequency, duration, and associations with individual and environmental factors. A retrospective study. Sleep Health. 2024;10(1 Suppl):S52–S62. doi:10.1016/j.sleh.2023.09.002
  3. Trotti LM. Waking up is the hardest thing I do all day: sleep inertia and sleep drunkenness. Sleep Med Rev. 2017;35:76–84. doi:10.1016/j.smrv.2016.08.005

General information, not medical advice. If you regularly sleep badly or feel sleepy during the day despite enough time in bed, talk to a doctor.

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Questions people ask

How does a sleep calculator work?

It adds or subtracts whole sleep cycles, about 90 minutes each on average, plus the time it takes you to fall asleep. The goal is an alarm that lands near the end of a cycle and enough cycles to reach 7 hours or more.

What time should I go to bed if I wake up at 6 AM?

With 90-minute cycles and 15 minutes to fall asleep: 8:45 PM for 6 cycles (9 hours), 10:15 PM for 5 cycles (7.5 hours), or 11:45 PM for 4 cycles (6 hours). Only the first two meet the 7-hour minimum for adults.

How long is a sleep cycle?

About 90 minutes on average, but it varies. A lab study of 369 healthy adults found a median of 96 minutes and a 95% range of 60 to 150 minutes, and the first cycle of the night was consistently the shortest.

Is 6 hours of sleep enough?

Not for most adults. The AASM and Sleep Research Society recommend 7 or more hours a night for adults aged 18 to 60, and regularly sleeping less is linked with obesity, diabetes, heart disease and depression.

Is it bad to wake up in the middle of a sleep cycle?

You may feel groggier, but grogginess on waking also depends on the time of night and how short of sleep you are. Getting enough total sleep matters more than hitting the end of a cycle to the minute.

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