How to use it
- Under What do you know?, pick I want to wake up at (bedtimes), I'm going to bed at (wake times), How long did I sleep? (bedtime and wake time) or Nap.
- Enter the time or times. To start from this moment, press Sleep now; in Nap mode it fills the nap start.
- Check Time to fall asleep (minutes). It starts at 15; set anything from 0 to 60. If you know your own cycle length, change Sleep cycle length (minutes) (60–120).
- Read the list of times for 3 to 6 cycles; the 5-cycle time is shown largest. Save it with Copy link, Save image or Share.
How the calculation works
The calculator counts back, or forward, in blocks of 90 minutes. For bedtimes: bedtime = wake time − time to fall asleep − n × 90 min. For wake times: wake time = bedtime + time to fall asleep + n × 90 min. Here n is 6, 5, 4 or 3 cycles.
The sleep itself is n × 90 minutes: 9 hours for 6 cycles, 7.5 hours for 5, 6 hours for 4 and 4.5 hours for 3. The time to fall asleep is added on top, because it is time in bed but not yet asleep.
| Cycles | Sleep time | Go to bed at |
|---|---|---|
| 6 | 9 h | 21:45 |
| 5 | 7.5 h | 23:15 |
| 4 | 6 h | 00:45 |
| 3 | 4.5 h | 02:15 |
How accurate is this?
The clock math is exact. What is not exact is the cycle: 90 minutes is an average, and real cycles change through the night and from person to person. That is why the cycle length can be edited. Waking at a cycle boundary is a rule of thumb, not something the calculator can check for you.
Nap mode shows alarms for a 10-minute nap, a 20-minute nap and one full cycle, each after the time to fall asleep. In a lab study of 24 young adults whose night had been cut to about 5 hours, Brooks and Lack (2006, Sleep) found that a 10-minute afternoon nap improved alertness and performance straight away. After a 20-minute nap the benefit appeared about 35 minutes later, and a 30-minute nap was followed by grogginess first.
Why there is a time to fall asleep
Lying down is not the same as being asleep. Without this field, every bedtime would be too late by however long you lie awake first.
The Sleep Foundation says a healthy person takes, on average, between 10 and 20 minutes to fall asleep. The 15-minute default sits in the middle of that range. A National Sleep Foundation expert panel (Ohayon and colleagues, 2017) counts how quickly you fall asleep, the sleep latency, among the indicators of good sleep quality.
If you know you usually take 5 or 30 minutes, change the number. Every time on the list moves by the same amount.
Why 90 minutes, and why it is only an average
During the night you move between non-REM and REM sleep several times. The National Institute of Neurological Disorders and Stroke (NINDS) says REM sleep first occurs about 90 minutes after falling asleep.
A full cycle is not a fixed length. SleepEducation.com puts each complete cycle at about 90 to 110 minutes, with most adults going through four to six cycles a night. NINDS adds that cycles change across the night: deep sleep comes in longer periods in the first half, and REM periods get longer later on.
So 90 minutes is a round average, not a timer. Your cycles vary within one night and differ from other people's. Treat the list as a set of reasonable options, not exact moments when a cycle ends.
How the times compare with sleep-duration guidance
The American Academy of Sleep Medicine and the Sleep Research Society recommend that adults aged 18 to 60 sleep 7 or more hours a night (2015 consensus). NINDS says most adults need 7 to 9 hours.
In the 07:00 example, the 5-cycle (7.5 h) and 6-cycle (9 h) bedtimes fall in that range. The 4-cycle and 3-cycle times are shorter than 7 hours. The list shows all of them so you can see the trade-off.
Bedtimes for 7, 8 and 9 hours of sleep
Common wake-up times with 15 minutes to fall asleep. The 5- and 6-cycle columns use 90-minute cycles; the 7 h and 8 h columns are plain hours of sleep.
| Wake up at | 7 h of sleep | 5 cycles (7 h 30 min) | 8 h of sleep | 6 cycles (9 h) |
|---|---|---|---|---|
| 05:30 | 22:15 | 21:45 | 21:15 | 20:15 |
| 06:00 | 22:45 | 22:15 | 21:45 | 20:45 |
| 06:30 | 23:15 | 22:45 | 22:15 | 21:15 |
| 07:00 | 23:45 | 23:15 | 22:45 | 21:45 |
| 07:30 | 00:15 | 23:45 | 23:15 | 22:15 |
| 08:00 | 00:45 | 00:15 | 23:45 | 22:45 |
Limits of a cycle calculator
Sleep cycles and your body clock are separate things. NINDS describes the circadian rhythm as a roughly 24-hour clock that sets daily patterns of alertness, body temperature and hormones. Sleep pressure, by contrast, builds the longer you stay awake.
A calculator can only count minutes. It doesn't know your body clock, how much sleep you missed earlier, or how often you wake in the night. Cycle lengths in children are also shorter than in adults, so the 90-minute block is for adults.
Planning training around your sleep? The heart rate zone calculator turns your age and resting heart rate into five zones.
Questions
- Why is the time to fall asleep set to 15 minutes?
- 15 minutes is in the middle of the 10 to 20 minutes a healthy person takes on average, according to the Sleep Foundation. You can change it anywhere from 0 to 60 in Time to fall asleep (minutes).
- Is every sleep cycle exactly 90 minutes?
- No. A complete cycle lasts about 90 to 110 minutes, and the stages inside it shift across the night. 90 minutes is used because it is a simple, common average.
- What does Sleep now do?
- It uses the current time as your bedtime and lists wake times for 3, 4, 5 and 6 cycles, with your time to fall asleep added.
- How many hours of sleep do adults need?
- The AASM and Sleep Research Society recommend 7 or more hours a night for adults 18 to 60. NINDS says most adults need 7 to 9 hours.
- If I go to bed at 23:00, which wake times does it show?
- With 15 minutes to fall asleep, sleep starts at 23:15. Five cycles end at 06:45, six cycles at 08:15 and four cycles at 05:15.
- Does the calculator save my times?
- No. Everything runs in your browser. Copy link keeps the times after a # in the link, which isn't sent to a server.
- How does "How long did I sleep?" work?
- It takes the time between bedtime and wake-up, crossing midnight if needed, and subtracts the time to fall asleep. 23:00 to 07:00 with 15 minutes to fall asleep is 7 h 45 min, about 5.2 cycles of 90 minutes.
- How long should a nap be?
- The calculator shows 10 minutes, 20 minutes and one full cycle. In Brooks and Lack's study of 24 adults, the 10-minute nap gave the quickest benefit. It is a study result, not a rule for everyone.