Sleep cycles explained: what time to go to bed
· 3 min read
You can sleep the same number of hours and wake up refreshed one morning and groggy for a long while the next. How you feel in the morning depends on when you wake up as much as on how long you slept. Here’s how sleep cycles work, and how to use them to figure out what time to go to bed.
Sleep runs in cycles
Sleep is made up of cycles that alternate between non-REM and REM sleep. Non-REM sleep is divided into lighter stages 1 and 2 and deep stage 3. After deep sleep, sleep becomes lighter again, and then REM sleep begins. The first REM period comes about 90 minutes after you fall asleep, and the cycle repeats several times a night.
The makeup of the cycles changes over the course of the night. Deep sleep is longer early on, and REM sleep gets longer as morning approaches.
Waking from deep sleep leaves you groggier
The state right after waking, when your head feels heavy and your thinking is slow, is called sleep inertia. Research suggests it’s stronger and lasts longer if you wake during deep sleep (slow-wave sleep). That’s where the idea of timing your wake-up for the end of a cycle, when sleep is lighter, comes from.
Count back from your wake-up time
- First, decide when you need to wake up. Say, 7:00 AM.
- Subtract five 90-minute cycles (7 hours 30 minutes) or six (9 hours).
- Then subtract the time it takes you to fall asleep (usually 10–20 minutes).
If you wake up at 7:00 AM and it takes you 15 minutes to fall asleep, go to bed at 11:15 PM for five cycles or 9:45 PM for six. Here’s a table for some common wake-up times (assuming 15 minutes to fall asleep).
| Wake-up time | 5 cycles (7 hr 30 min) | 6 cycles (9 hr) |
|---|---|---|
| 6:00 AM | 10:15 PM | 8:45 PM |
| 6:30 AM | 10:45 PM | 9:15 PM |
| 7:00 AM | 11:15 PM | 9:45 PM |
| 7:30 AM | 11:45 PM | 10:15 PM |
For other times, try the Haruda sleep calculator. You can adjust the time it takes to fall asleep there, too.
How many cycles do you need?
The National Sleep Foundation (NSF) recommends 7–9 hours a day for adults aged 18–64 and 8–10 hours for teens aged 14–17. For adults, five cycles (7 hours 30 minutes) or six (9 hours) fall within that range. Waking after four cycles (6 hours) may feel easier in the moment, but it won’t make up for the sleep you’re missing.
Some days the math won’t add up
- Cycle length varies from person to person, and even from night to night for the same person. It ranges from roughly 70 to 120 minutes.
- How long it takes to fall asleep depends on that day’s fatigue, caffeine, and stress.
- So treat 90 minutes as a starting point, not the right answer.
For about a week, write down when you fall asleep and when you wake up. Then use a morning you woke up refreshed as your baseline, and try moving your bedtime earlier or later in 15-minute steps. The time you land on is the closest match to your own cycle.
Your records have the answer
With Haruda Alarm, you can log your wake-up time and bedtime with a single tap and see them in a weekly report. Before bed, it tells you how many hours you’ll sleep if you fall asleep now, and it can remind you to log your sleep at a time you choose, so you don’t miss your bedtime.