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Sleep Calculator

Plan your night around whole sleep cycles: enter the time you need to get up to see when to go to bed, or the time you're going to bed to see when to set the alarm.

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How to use the sleep calculator

  1. Choose When to go to bed if you know when you have to get up, or When to wake up if you know when you're going to bed.
  2. Enter the wake-up time or the bedtime.
  3. Check the time to fall asleep. It starts at 15 minutes; change it if you usually drift off faster or slower.
  4. Check the sleep cycle length. It starts at 90 minutes, a common average.
  5. Read the suggested time for 5 cycles, and the table of options for 4, 5 and 6 cycles.

How the times are calculated

During the night you move through repeating cycles of light sleep, deep sleep and REM (dreaming) sleep. The calculator times your alarm for the end of a cycle, when sleep is lighter:

Bedtime = wake-up time − (cycles × cycle length) − time to fall asleep Wake time = bedtime + time to fall asleep + (cycles × cycle length) Defaults: cycle length = 90 minutes, time to fall asleep = 15 minutes 4 cycles = 6 h 5 cycles = 7 h 30 min 6 cycles = 9 h of sleep

The NIH's StatPearls review of sleep stages describes a typical night as four to five cycles of roughly 90 to 110 minutes each. Cycles also change through the night: deep sleep dominates the early ones and REM sleep gets longer toward morning. The 90-minute figure is an average, not a rule.

Worked example

The calculator opens with a 7:00 AM wake-up time, 15 minutes to fall asleep and 90-minute cycles:

  • 5 cycles: 5 × 90 = 450 minutes, or 7 h 30 min of sleep.
  • To be asleep by 7:00 AM − 7 h 30 min = 11:30 PM, you need to be in bed 15 minutes earlier: 11:15 PM.
  • 6 cycles (9 hours): 7:00 AM − 9 h − 15 min = 9:45 PM.
  • 4 cycles (6 hours): 7:00 AM − 6 h − 15 min = 12:45 AM.

The other way round, going to bed at 10:30 PM means falling asleep around 10:45 PM. Adding 5 cycles gives a wake-up time of 6:15 AM, with 4:45 AM for 4 cycles and 7:45 AM for 6.

How much sleep you need

The calculator suggests 5 cycles because 7.5 hours meets the adult guideline. These are the CDC's recommended amounts:

Age Recommended sleep
6–12 years 9–12 hours
13–17 years 8–10 hours
18–60 years 7 or more hours
61–64 years 7–9 hours
65 years and older 7–8 hours

Source: CDC, About Sleep. Needs vary from person to person, so use the table as a guide. Teenagers often need 6 cycles to reach 8 hours or more, and 4 cycles (6 hours) is short for nearly everyone if it happens night after night.

Getting the most from the result

  • Find your own cycle length. For a week, note when you wake up naturally without an alarm. If it's usually around 7 h 50 min after falling asleep, your cycles may be closer to 95 or 100 minutes. Enter that instead of 90.
  • Be honest about falling asleep. If you read or scroll in bed, count that time too, or you'll land in the middle of a cycle.
  • Keep it regular. Going to bed and getting up at about the same time every day, including weekends, matters more than hitting an exact minute.

Feeling tired most days even after enough sleep, loud snoring, or stopping breathing in your sleep are reasons to see a doctor.

Frequently asked questions

What time should I go to bed to wake up at 7 AM?

With 90-minute cycles and 15 minutes to fall asleep, aim for 11:15 PM for 7.5 hours of sleep or 9:45 PM for 9 hours. If 11:15 PM is too late for you, 9:45 PM is the next whole-cycle bedtime.

Why do I feel worse after 8 hours than after 7.5?

An alarm that goes off in deep sleep can leave you groggy for a while, a feeling called sleep inertia. With 90-minute cycles, 8 hours ends partway through the sixth cycle, while 7.5 hours ends close to the end of the fifth. Cycle length varies, though, so this won't hold for everyone.

Is 6 hours of sleep enough?

For most adults, no. The CDC recommends at least 7 hours for adults aged 18 to 60. Six hours fits 4 whole cycles, so you may wake up easily, but you'll still build up a sleep debt if you do it night after night.

Does the calculator work for naps?

The same idea applies. A nap of about 20 minutes usually stays in light sleep, while about 90 minutes lets you finish a whole cycle. Naps in between are more likely to end in deep sleep and leave you groggy for a while.

Related calculators

  • Age Calculator — Your exact age in years, months and days, with totals in weeks, days and hours and the date of your next birthday.
  • Time Calculator — Add or subtract days, hours, minutes and seconds from a time, combine two durations, or convert between time units.
  • BMR Calculator — Your basal metabolic rate from the Mifflin-St Jeor, revised Harris-Benedict and Katch-McArdle equations.

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