Deep sleep (N3 slow-wave sleep) is the deepest non-REM stage, characterized by slow brain waves, lowest heart rate, and deepest restoration.
Mechanism: Growth hormone pulses peak. The glymphatic system clears metabolic waste from the brain. Memory consolidation. Muscle protein synthesis. Physical repair dominates.
Use case: Most adults get 60-90 min per night. Optimize via cool bedroom (65-68°F), magnesium glycinate, no alcohol within 3 hours of bed, last meal 3+ hours before sleep.
Caveats: Sleep trackers underestimate deep sleep significantly. Use 7-day rolling averages, not single nights. Alcohol crushes deep sleep. That's why you feel destroyed after even moderate drinking.