Guide: sleep cycles

Sleep is not uniform — it moves in ~90-minute cycles. Understanding this changes how you plan bedtime and alarms.

What is a sleep cycle?

Each cycle includes several stages:

  • N1 — light sleep, falling asleep (5–10 min)
  • N2 — lighter sleep (~45–50% of night)
  • N3 — deep sleep, physical recovery
  • REM — dreams, memory consolidation

Why 90 minutes?

Average cycle length is 90 min (80–120 min range). Waking at cycle end — during light sleep — reduces sleep inertia.

CycleZen lets you adjust cycle length in all calculators.

Try the wake-up calculator