How the Sleep Calculator Works
A sleep cycle lasts about 90 minutes, and waking at the end of one, when sleep is lightest, is much easier than waking from deep sleep. The calculator counts back from your wake-up time in 90-minute cycles, plus 15 minutes to fall asleep. For a 7:00 AM alarm, that means going to bed at 9:45 PM for six cycles (9 hours) or 11:15 PM for five cycles (7.5 hours).
Enter a bedtime instead and it counts forward to give you wake-up times. On the results screen, you can change the 15 minutes to however long you usually take to fall asleep.
Bedtimes for Common Wake-Up Times
These bedtimes include 15 minutes to fall asleep.
| Wake up at | 6 cycles (9 h) | 5 cycles (7.5 h) | 4 cycles (6 h) |
|---|---|---|---|
| 5:00 AM | 7:45 PM | 9:15 PM | 10:45 PM |
| 5:30 AM | 8:15 PM | 9:45 PM | 11:15 PM |
| 6:00 AM | 8:45 PM | 10:15 PM | 11:45 PM |
| 6:30 AM | 9:15 PM | 10:45 PM | 12:15 AM |
| 7:00 AM | 9:45 PM | 11:15 PM | 12:45 AM |
| 7:30 AM | 10:15 PM | 11:45 PM | 1:15 AM |
| 8:00 AM | 10:45 PM | 12:15 AM | 1:45 AM |
| 9:00 AM | 11:45 PM | 1:15 AM | 2:45 AM |
Most adults do best with five or six cycles. Four is fine for an occasional short night, but not every night.
How Sleep Cycles Work
Through the night you move through light sleep, deep sleep and REM sleep, then start again. Each round is one sleep cycle, and most adults go through four to six a night. Deep sleep fills more of the early cycles, and REM sleep more of the later ones.
When an alarm goes off in the middle of deep sleep, you wake up groggy and disoriented, a state called sleep inertia. Timing your alarm for the end of a cycle makes it more likely you wake from light sleep instead. Read more about how long a sleep cycle is.
What the Calculator Can't Know
Ninety minutes is an average. Real cycles vary from person to person and through the night, roughly between 70 and 120 minutes, so treat these times as a good starting point rather than an exact prediction. If you still wake up tired after a full night, the cause may be something other than timing.
Frequently Asked Questions
How long is a sleep cycle?
About 90 minutes on average, though it varies between roughly 70 and 120 minutes.
How many sleep cycles do I need?
Five or six for most adults, which is 7.5 to 9 hours. The CDC recommends that adults get at least 7 hours of sleep a night.
Is it better to sleep 6 hours or 7.5 hours?
7.5 hours. Both end at the end of a cycle, but 6 hours is less than the 7 hours adults need, so keep it for the occasional short night.
What if I can't fall asleep in 15 minutes?
On the results screen, change the minutes to how long you usually take. The times update straight away.
Can I set these times as my alarm?
Yes. In the FixSleep app, enter your bedtime and tap one of the suggested wake-up times to set it as your alarm. To turn it off in the morning, you complete a mission, like solving math problems or scanning a QR code in another room.