Sleep Stages

Sleep stages classify sleep into light sleep (N1/N2), deep sleep (N3/slow-wave), and REM (rapid eye movement) sleep. A typical night cycles through stages 4-6 times, with more deep sleep early and more REM later. Deep sleep is critical for physical restoration, immune function, and memory consolidation. REM sleep supports emotional processing, learning, and creativity. Adults should get 1-2 hours of deep sleep and 1.5-2 hours of REM per night. Wearables estimate sleep stages using heart rate variability patterns and movement, but accuracy is limited compared to polysomnography (the clinical gold standard using EEG). Consumer devices may misclassify stages by 20-30%. Trends are more reliable than individual night data. Alcohol, medications, and sleep disorders significantly alter stage distribution.