Sleep isn't one long block of rest. Through the night your brain moves through repeating sleep cycles, each lasting roughly 90 minutes. Knowing how many you need, and when each one ends, is the key to waking up refreshed.
Want your exact times?
Open the sleep calculatorWhat happens in a sleep cycle
Each cycle moves through the same stages:
- N1, drifting off: a few minutes of very light sleep.
- N2, light sleep: your heart rate slows and body temperature drops. This takes up about half the night.
- N3, deep sleep: the most restorative stage, when your body repairs itself. It's hardest to wake from.
- REM sleep: when most dreaming happens. It helps with memory and mood.
Early in the night your cycles contain more deep sleep. Later cycles have longer REM periods, which is why you often wake from a dream in the morning.
How many cycles by age
| Age group | Recommended sleep | Cycles |
|---|---|---|
| Teenagers (13–18) | 8–10 hours | About 6 |
| Adults (18–60) | 7 or more hours | 5–6 |
| Older adults (61+) | 7–9 hours | 5–6 |
Does every cycle last 90 minutes?
No. Cycles usually last between 80 and 110 minutes and change through the night. Ninety minutes is a useful average for planning, but if you often wake up groggy, try moving your bedtime 10 to 15 minutes earlier or later.
How to use this
Pick your wake-up time, count back five or six cycles, and add the 10 to 20 minutes it takes you to fall asleep. That's your ideal bedtime. The sleep calculator on our homepage does this for you instantly.