Sleep Calculator
Find the best bedtime or wake-up time based on 90-minute sleep cycles.
Sleep happens in cycles of about 90 minutes — waking up between cycles, rather than in the middle of one, tends to feel more refreshing.
Go to bed at one of these times
Assumes about 14 minutes to fall asleep — adjust the time yourself if you know you take longer.
About this calculator
What Is a Sleep Calculator?
A sleep calculator suggests the best times to go to bed or wake up so that you finish a full sleep cycle instead of being pulled out of deep sleep. Tell it when you need to wake up and it lists the bedtimes that fit; or tell it when you are going to bed and it lists the times to set your alarm. Waking at the end of a cycle usually feels better than waking in the middle of one, even if the total sleep is the same.
What Are Sleep Cycles?
During the night your brain moves through repeating cycles that alternate between non-REM sleep and REM (rapid eye movement) sleep. A typical cycle lasts about 90 minutes, and a night has four to six of them. Each cycle contains:
- Light sleep (N1 and N2): you drift off, your heart rate slows and body temperature drops.
- Deep sleep (N3): the most restorative stage for the body, strongest in the first half of the night — tissue repair, growth and immune support happen here.
- REM sleep: dreaming, memory processing and learning; REM periods get longer toward morning.
The calculator also adds the time most people need to fall asleep, usually around 15 minutes, which you can adjust to match your habits.
Worked Example
To wake up at 7:00 AM, going to bed at 9:46 PM allows for 6 full cycles (9 hours), 11:16 PM allows for 5 cycles (7.5 hours), and 12:46 AM allows for 4 cycles (6 hours).
How Much Sleep Do You Need?
- Adults (18–64): 7–9 hours a night.
- Older adults (65+): 7–8 hours.
- Teenagers: 8–10 hours.
- School-age children: 9–12 hours.
These are general ranges; what matters is how you feel in the morning and whether you stay alert through the day.
Tips for Better Sleep
- Keep a regular schedule, even on weekends, so your body clock stays steady.
- Keep the bedroom cool, dark and quiet, and put screens away an hour before bed.
- Limit caffeine in the afternoon and heavy meals or alcohol close to bedtime.
- Get daylight in the morning and exercise during the day.
The circadian clock responds to light, which is why jet lag and shift work disrupt sleep. Persistent trouble sleeping is worth discussing with a doctor. To measure the length of a night, try the Time Duration Calculator.
Worked example
To wake up at 7:00 AM, going to bed at 9:46 PM allows for 6 full sleep cycles (9 hours) plus about 14 minutes to fall asleep.
Frequently asked questions
Why 90-minute cycles?
A full sleep cycle — moving through light sleep, deep sleep, and REM — takes roughly 90 minutes on average. Waking up at the end of a cycle rather than in the middle of one (especially deep sleep) tends to feel less groggy.
Is 90 minutes exact for everyone?
No — individual cycle length varies, typically 70 to 120 minutes. This calculator uses the commonly cited 90-minute average as a starting point, not a personalized measurement.
Why does it add 14 minutes?
That's a typical estimate for how long it takes an average person to fall asleep after getting into bed. Adjust your actual bedtime earlier if you know you take longer to fall asleep.
