Sleep cycles explained
Sleep isn't a uniform block — it unfolds in successive ~90-minute cycles, each with distinct phases.
Advertisement
The 4 phases of a sleep cycle
Falling asleep (N1): a few minutes, very light sleep. Light sleep (N2): most of total sleep time. Deep sleep (N3): physical recovery, the most restorative phase but the hardest to wake from. REM sleep: intense brain activity, dreams, memory consolidation. Adults go through 4 to 6 cycles per night; early cycles have more deep sleep, later ones more REM.
Frequently asked questions
How many cycles per night?
4 to 6 on average. Aiming for 5-6 complete cycles (7.5 to 9 hours) suits most adults.
Why does cycle length vary slightly?
90 minutes is an average — depending on the person, age and fatigue, a cycle can last 70 to 120 minutes.
Want to go further?
Explore our guides and other calculators.