Sleep Calculator

How this sleep calculator works

Sleep comes in cycles. In each one you move through light sleep, deep sleep, and REM sleep, then start again. This calculator uses 90 minutes as the average cycle and adds the time it takes you to fall asleep: 15 minutes unless you enter your own.

• From a wake-up time, it counts back whole cycles to find your bedtimes.
• From a bedtime, it counts forward to find your wake-up times. It starts from the current time, so you can check right before bed.

Bedtime = wake-up time − (cycles × 90 minutes) − time to fall asleep

Aim for 5 or 6 cycles: 7.5 or 9 hours of sleep. Adults need at least 7 hours a night, so 3 or 4 cycles (4.5 or 6 hours) works for a night, not as a habit.

What time should I go to bed?

Bedtimes for common wake-up times, with 15 minutes to fall asleep:

If you usually take longer to fall asleep, move your bedtime earlier by the difference.

When should I wake up?

Wake-up times for common bedtimes, with 15 minutes to fall asleep:

How long is a sleep cycle?

About 90 minutes on average, but it varies. The National Heart, Lung, and Blood Institute puts a full cycle at 80 to 100 minutes, with four to six cycles a night. You may wake briefly between cycles without remembering it.

Cycles also change as the night goes on. Your first REM sleep starts about 90 minutes after you fall asleep. Deep sleep comes in longer stretches in the first half of the night, and REM periods get longer toward morning.

Because real cycles vary, treat the times above as a guide, not a schedule to the minute.

Why wake up at the end of a cycle?

Waking up groggy has a name: sleep inertia. It's worst when an alarm pulls you out of deep sleep and mildest from light sleep, with REM in between. Being short on sleep makes it worse, partly because it gives you more deep sleep. The fog lifts fast at first, but full alertness can take an hour or more.

Timing your alarm to the end of a cycle aims for lighter sleep. It can't guarantee it, since cycles vary. Getting enough sleep matters more than hitting the exact minute.

How much sleep do you need?

The National Sleep Foundation's recommended ranges:

The American Academy of Sleep Medicine and the Sleep Research Society agree for adults: at least 7 hours a night, regularly. In cycles, that's 5 cycles (7.5 hours) for most adults. 6 cycles (9 hours) is the top of the adult range and suits teens.

Sleep stages on Apple Watch

Apple Watch estimates your sleep stages each night: Awake, REM, Core, and Deep. Core is Apple's name for light sleep, and it's where you spend most of the night. To see last night's stages, open the Health app and search for Sleep.

On the stages chart you can usually spot the cycles: more deep sleep early in the night, longer REM toward morning. Stage estimates from any wearable are approximate. Our guide to Apple Watch sleep tracking accuracy covers how close they get.

Track your sleep with Ness

A calculator gives you a plan. Ness shows how the night actually went, from your Apple Watch sleep data in Apple Health. It adds:

• Sleep score: out of 100, from how long you slept, your deep and REM sleep, how often you woke up, how much of your time in bed you spent asleep, and how far your heart rate dropped.
• Bedtime consistency: how far last night's bedtime drifted from your usual one.
• Sleep reminders: optional nudges to wind down and go to bed.
• Circadian Rhythm: a planning estimate of your day, from your first energy peak and afternoon dip to when to wind down, based on last night's sleep.

Ness is an AI health coach for iPhone and Apple Watch. It also scores your recovery, strain, and stress every day, logs meals from a photo, and answers questions about your own data.

FAQ

What time should I go to bed if I wake up at 6? To wake up at 6:00 AM, go to bed at 10:15 PM for 5 sleep cycles (7.5 hours of sleep) or 8:45 PM for 6 cycles (9 hours). Both allow 15 minutes to fall asleep. If you take longer, enter your own time in the calculator.

If I go to bed now, what time should I wake up? Switch the calculator to "Go to bed at". It starts from the current time and shows wake-up times for 3 to 6 cycles. For example, going to bed at 11:00 PM and falling asleep in 15 minutes, 5 cycles ends at 6:45 AM and 6 cycles at 8:15 AM.

How long is a sleep cycle? About 90 minutes on average. The National Heart, Lung, and Blood Institute puts a full cycle at 80 to 100 minutes. Cycles vary from person to person and through the night.

How many sleep cycles do I need each night? Most people get 4 to 6 cycles a night. Five cycles is 7.5 hours of sleep and six is 9 hours, both within the 7 to 9 hours recommended for adults.

Is it better to wake up at the end of a sleep cycle? It can help. Waking from deep sleep leaves you groggier than waking from light sleep. But cycle lengths vary, so a calculator can only aim for the end of a cycle. Getting enough total sleep matters more.

How much sleep do I need for my age? The National Sleep Foundation recommends 7 to 9 hours for adults 18 to 64, 7 to 8 hours from 65, 8 to 10 hours for teens 14 to 17, and 9 to 11 hours for children 6 to 13.

Is 10pm to 7am enough sleep? Yes, for most adults. It's 9 hours in bed. Even after time to fall asleep and brief wake-ups, that usually leaves more than the 7 hours adults need.

What is the 10-3-2-1-0 sleep rule? It's a popular rule of thumb, not a clinical guideline: no caffeine in the 10 hours before bed, no food or alcohol in the last 3 hours, no work in the last 2, no screens in the last hour, and zero taps of the snooze button. To use it, count back from the bedtime this calculator gives you.

How long should it take to fall asleep? The National Sleep Foundation counts falling asleep within 30 minutes as a sign of good sleep for adults. This calculator assumes 15 minutes unless you enter your own.

Does this calculator save my data? It runs in your browser and sends nothing anywhere. It remembers your wake-up time on this device so it's ready next time.

Related reading

• Sleep Debt: What the Research Says About Paying It Back
• Apple Watch Sleep Score: What's Good, What It Misses, and How to Use It
• How Accurate Is Apple Watch Sleep Tracking, Really?
• Caffeine Calculator
• Biological Age Calculator

Sources

1. National Heart, Lung, and Blood Institute. How Sleep Works: Sleep Phases and Stages.
2. National Institute of Neurological Disorders and Stroke. Brain Basics: Understanding Sleep.
3. Hirshkowitz M, et al. National Sleep Foundation's sleep time duration recommendations: methodology and results summary. Sleep Health, 2015.
4. Watson NF, 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. Journal of Clinical Sleep Medicine, 2015.
5. National Sleep Foundation. What Is Sleep Quality?
6. Tassi P, Muzet A. Sleep inertia. Sleep Medicine Reviews, 2000.
7. Hilditch CJ, McHill AW. Sleep inertia: current insights. Nature and Science of Sleep, 2019.
8. Apple. Estimating Sleep Stages from Apple Watch. 2025.

This calculator is for general wellness and education. It isn't medical advice. If you often struggle to sleep, or feel sleepy during the day after enough time in bed, talk to your doctor.