Feature

Sleep Staging

Classification of sleep into distinct stages—typically Wake, Light (N1/N2), Deep (N3/slow-wave), and REM—based on physiological signals during sleep. Clinical polysomnography (PSG) uses EEG, EOG, and EMG to stage sleep; wearables approximate staging using heart rate patterns, HRV, movement, and sometimes respiratory rate and SpO2. Each stage serves distinct functions: light sleep facilitates memory consolidation and transition, deep sleep enables physical restoration, hormone release (growth hormone), and immune function, while REM supports cognitive processing, emotional regulation, and memory integration. Healthy adults typically experience 4-6 sleep cycles nightly, with deep sleep concentrated early and REM increasing toward morning. Wearable staging accuracy varies significantly compared to PSG gold standard—deep sleep detection is particularly challenging—but relative trends and patterns provide useful insights. Sleep staging helps users understand sleep architecture, identify factors affecting specific stages (alcohol reduces REM, late exercise may reduce deep sleep), and optimize sleep quality beyond simple duration.

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