How to use
Pick a mode with the tabs at the top.
- From wake-up time: enter the time you need to get up and you’ll see bedtimes that allow 4–6 cycles of sleep.
- From bedtime: enter the time you go to bed, or tap “Now”, and you’ll see wake-up times after 4–6 cycles.
The large time at the top is the latest bedtime (or earliest wake-up time) that still gives you 7 hours of sleep or more. The table below lists the time and total sleep for each number of cycles, and rows under 7 hours are marked “Shorter than recommended”. Open “Options” to change the cycle length (90 min by default, 70–120 min), the time to fall asleep (15 min by default, 0–60 min) and the length of the first cycle.
What is a sleep cycle?
Sleep alternates between non-REM sleep (stages 1–3) and REM sleep, and one round of this is called a sleep cycle. According to the sleep medicine textbook by Carskadon and Dement (2011), the first cycle averages about 70–100 minutes and later cycles about 90–120 minutes. Deep (slow-wave) sleep is concentrated in the first third of the night, and REM periods get longer toward morning. REM sleep makes up about 20–25% of total sleep.
“90 minutes” is a rounded figure for these values. A 2024 study (Cajochen et al.) that analyzed 6,064 cycles from sleep lab recordings (polysomnography) of 369 people found a median cycle length of 96 minutes, large differences between people, and a first cycle shorter than later ones. Age, sex and the sleep pressure built up before bedtime also affected cycle length.
How it’s calculated, and its limits
Total sleep is “number of cycles × cycle length” (if you set the first cycle separately, only the first cycle uses that value). Adding the time it takes to fall asleep gives your time in bed.
For example, if you need to get up at 7:00 a.m., with 90-minute cycles and 15 minutes to fall asleep, the estimated bedtime is 11:15 p.m. for 5 cycles (7 hr 30 min of sleep), 9:45 p.m. for 6 cycles (9 hr) or 12:45 a.m. for 4 cycles (6 hr). Four cycles is under 7 hours, so it is marked “Shorter than recommended”. The guideline check uses total sleep time, not the number of cycles — with 120-minute cycles, even 4 cycles is 8 hours.
The results are estimates. Cycle length differs between people and from night to night, and if you wake up during the night or fall asleep later than expected, your actual cycles will drift from the calculation. The calculation deals only with times of day and doesn’t show dates.
The highlighted time is selected from the 4–6-cycle candidates; it is not a measurement of your ideal sleep schedule. The default 15 minutes to fall asleep is also a planning assumption, so adjust it to your usual experience.
Recommended sleep duration
The 2015 joint consensus of the American Academy of Sleep Medicine (AASM) and the Sleep Research Society (SRS) (Watson et al.) recommends that adults aged 18–60 regularly sleep 7 hours or more per night for good health. The “recommended” marks in this calculator follow this guideline. The 2015 National Sleep Foundation (NSF) recommendations (Hirshkowitz et al.) give these ranges by age:
| Age | Recommended sleep |
|---|---|
| School-age children | 9–11 hours |
| Teenagers | 8–10 hours |
| Young adults and adults | 7–9 hours |
| 65 and older | 7–8 hours |
Both are expert consensus statements from US scientific organizations. Timing a short night to your cycles is no substitute for getting enough sleep.
Signs you should see a sleep specialist
If any of these keep happening, talk to a sleep specialist rather than just adjusting your times with a calculator:
- It often takes you more than 30 minutes to fall asleep.
- You feel very sleepy during the day even after enough sleep.
- You snore heavily, or someone tells you that you stop breathing in your sleep.
- You find it very hard to get up.
This calculator doesn’t replace diagnosis or treatment.