Understanding the 5 Stages of Sleep: A Comprehensive Guide

Sleep architecture cycles through NREM Stages N1, N2, N3 and REM — not 5 stages (older R&K classification). Modern AASM (2007) defines 4 stages:

Current Sleep Stages (AASM)

1. Stage W (Wakefulness): Alpha (eyes closed), beta (eyes open).
2. N1 (Light Sleep, 5%): Theta, slow eye movements, vertex waves. Transition; easily aroused.
3. N2 (Deeper Light Sleep, 45–50%): Sleep spindles (12–16 Hz), K-complexes. No eye movement. Memory consolidation, synaptic downscaling.
4. N3 (Slow-Wave / Deep Sleep, 15–25%): ≥20% delta waves (0.5–2 Hz, >75 μV). Growth hormone release, tissue repair, immune function, glymphatic clearance. Hard to arouse.
5. REM (Rapid Eye Movement, 20–25%): Low-voltage mixed-frequency EEG, sawtooth waves, rapid eye movements, muscle atonia. Vivid dreaming, emotional processing, memory integration. Cycle length ~90 min; REM proportion increases across night.

Functions

• N3: Physical restoration, growth, immune, metabolic clearance.
• REM: Emotional regulation, creativity, procedural memory, synaptic homeostasis.
• N2: Declarative memory consolidation, spindle-dependent learning.

Clinical Relevance

• Sleep disorders alter architecture: apnea (fragmentation, ↓N3/REM), insomnia (↑N1, ↓N3), narcolepsy (SOREMPs), depression (↓REM latency).
• Aging: ↓N3, ↑fragmentation, phase advance.

Healthy sleep = adequate duration, continuity, and stage distribution.