Sleep Cycle

A 90-minute pattern of four sleep stages that repeats 4-6 times per night.

Each cycle includes: N1 (light sleep, 5 min), N2 (moderate sleep, 25 min), N3 (deep sleep, 30 min), and REM (dream sleep, 20-25 min). Waking during deep sleep or REM causes grogginess. Waking between cycles — during the brief light phase — feels natural. This is why 7.5 hours (5 complete cycles) can feel better than 8 hours (5 cycles plus 30 minutes into a 6th).

Try it yourself: