Help & sleep guide

A free online sleep calculator based on 90-minute REM sleep cycles. Find the best time to wake up if you go to sleep now, or when to go to bed for your alarm.

Open the sleep calculator

REM sleep calculator: how sleep cycles work

Every night you move through several sleep cycles, each lasting roughly 90 minutes. A cycle passes from light sleep into deep sleep and then into REM sleep, where most dreaming happens.

Waking up in the middle of deep sleep is what makes you feel groggy (sleep inertia). Waking at the end of a cycle, when sleep is lightest, helps you feel alert, even if you slept a little less.

AwakeREMN1N2N3
N1Light sleep, a few minutes. Easy to wake.
N2Heart rate slows, body temperature drops. About half your night.
N3Deep sleep. Physical recovery and growth hormone. Hardest to wake from.
REMDreaming, memory and emotional processing. Gets longer toward morning.

Sleep calculator hours chart

Bedtimes for common wake-up times, assuming 15 minutes to fall asleep. Use it as a quick alarm clock calculator.

CyclesSleep6:00 AM6:30 AM7:00 AM7:30 AM
69 h8:45 PM9:15 PM9:45 PM10:15 PM
57.5 h10:15 PM10:45 PM11:15 PM11:45 PM
46 h11:45 PM12:15 AM12:45 AM1:15 AM
34.5 h1:15 AM1:45 AM2:15 AM2:45 AM

How many hours of sleep do you need?

Recommended sleep per day by age group (National Sleep Foundation guidelines).

Age groupRecommended
Newborn (0–3 months)14–17 h
Infant (4–11 months)12–15 h
Toddler (1–2 years)11–14 h
Preschool (3–5 years)10–13 h
School age (6–13 years)9–11 h
Teen (14–17 years)8–10 h
Young adult (18–25)7–9 h
Adult (26–64)7–9 h
Older adult (65+)7–8 h

Frequently asked questions

How does the sleep calculator work?

A typical sleep cycle lasts about 90 minutes. This online sleep calculator counts back (or forward) in 90-minute cycles from your target time and adds the average time it takes to fall asleep, so your alarm goes off at the end of a cycle instead of during deep sleep.

If I go to sleep now, when should I wake up?

Add about 15 minutes to fall asleep, then 4.5, 6, 7.5 or 9 hours (3 to 6 full sleep cycles). For example, if you go to bed at 11:00 PM, good wake-up times are 6:45 AM (5 cycles) or 8:15 AM (6 cycles). Use the “If I go to sleep now” tab on the calculator to calculate it from the current time.

What is a REM sleep calculator?

A REM sleep calculator times your alarm to the end of a sleep cycle, right after a REM phase, when sleep is lightest. Waking there feels much easier than waking from deep sleep. This calculator works the same way, using an average 90-minute cycle.

How many hours of sleep do I need?

Most adults do best with 7 to 9 hours, which is 5 to 6 sleep cycles (7.5 or 9 hours). Four cycles (6 hours) is fine occasionally, but regularly sleeping less than 7 hours builds up sleep debt. See the sleep hours chart above for exact times.

Can I use this as an alarm clock calculator?

Yes. Enter the time you go to bed in the “I want to go to bed at” tab and the sleep alarm calculator shows the best times to set your alarm clock. Tap a time to copy it.

Is this a deep sleep calculator?

Indirectly. Deep sleep (N3) happens mostly in the first half of the night, and you cannot predict its exact minutes without a tracker. What this calculator does is help you avoid waking up during deep sleep, which is what causes grogginess.

How long does it take to fall asleep?

Most healthy adults fall asleep in 10 to 20 minutes. The calculator uses this 15-minute average, so every time it shows already includes it.

Is every sleep cycle exactly 90 minutes?

No. Cycles range from about 70 to 120 minutes and vary between people and across the night. 90 minutes is a good average. If you consistently wake up groggy, try shifting your alarm by 10 to 15 minutes.

What is the best nap length?

A 20-minute power nap boosts alertness without entering deep sleep. A 90-minute nap covers a full cycle including REM. Naps of 30 to 60 minutes often end in deep sleep and leave you groggy.