The Architecture of Sleep

Rest is not one uniform state. Your brain cycles through distinct stages roughly every 90 minutes. Each cycle includes light sleep, deep sleep, and REM (rapid eye movement) sleep. A full night usually contains four to six complete cycles, and every stage serves a different biological purpose.

Light sleep (stages 1-2) is the transition zone where muscles relax and heart rate slows — about 50% of the night. Deep sleep (stage 3) triggers growth hormone surges, tissue repair, and immune system strengthening.