How sleep cycles work
Each night you move through four or six cycles. Each cycle passes through light sleep, deep (slow-wave) sleep and REM sleep, where most dreaming happens. Early cycles contain more deep sleep, which restores the body; later cycles contain more REM, which matters for memory and mood.
Waking during deep sleep causes "sleep inertia": that heavy, confused feeling that can last half an hour. Waking near the end of a cycle, during light sleep, usually feels easier.
How the times are calculated
The average adult takes 10–20 minutes to fall asleep. Change the setting if you know you're faster or slower.
How much sleep do you need?
| Age | Recommended per night |
|---|---|
| Teenagers (14–17) | 8–10 hours |
| Adults (18–64) | 7–9 hours |
| Older adults (65+) | 7–8 hours |
Source: US National Sleep Foundation. Five cycles (7.5 hours) or six (9 hours) fit these ranges best.
Cycles aren't exactly 90 minutes
Real cycles vary between about 70 and 120 minutes, from person to person and through the night. Treat the times as a good starting point: if you still wake groggy, try shifting your bedtime 15 minutes earlier or later for a week.
Tips for better sleep
- Keep the same wake-up time every day, including weekends.
- Get daylight in the morning; dim the lights and screens in the evening.
- Avoid caffeine after early afternoon and big meals or alcohol close to bedtime.
- Keep the bedroom cool, dark and quiet.
Frequently asked questions
What time should I go to bed to wake up at 6 am?
Aim to be asleep by 9:00 pm (6 cycles) or 10:30 pm (5 cycles). Add about 15 minutes to fall asleep: go to bed around 8:45 pm or 10:15 pm.
How long is a sleep cycle?
About 90 minutes on average, although it varies from about 70 to 120 minutes.
Is 6 hours of sleep enough?
For most adults, no. Six hours is four cycles; adults generally need 7–9 hours.
Why do I feel tired after 8 hours of sleep?
You may be waking in the middle of a deep-sleep cycle. Try going to bed 15–30 minutes earlier or later.
Does this work for naps?
Short naps of 20 minutes avoid deep sleep. A 90-minute nap covers one full cycle.
Last updated October 2026 · More health & fitness calculators