Deep sleep, also known as slow-wave sleep (SWS) or stage N3 sleep, is the most physically restorative stage of the sleep cycle. It is characterized by very slow, high-amplitude brain waves called delta waves, minimal eye movement, reduced heart rate and blood pressure, and significantly decreased responsiveness to external stimuli. During deep sleep, the body is at its most relaxed and hardest to wake — if roused from deep sleep, a person typically feels disoriented and groggy, a phenomenon known as sleep inertia. Deep sleep occurs predominantly in the first half of the night and during the earlier sleep cycles. As the night progresses, sleep cycles shift toward more REM sleep and lighter non-REM sleep. Adults typically spend about 15 to 20 percent of their total sleep time in deep sleep, though this proportion decreases with age. The functions of deep sleep are critical to health. During deep sleep, the body releases growth hormone, which is essential for tissue repair, muscle growth, and cell regeneration. The immune system is strengthened, and the brain clears metabolic waste products — including amyloid-beta, a protein associated with Alzheimer’s disease — through the glymphatic system. Memory consolidation, particularly for declarative (factual) memories, also occurs during deep sleep. Factors that reduce deep sleep include aging, alcohol consumption (which suppresses deep sleep despite initially promoting sleep onset), sleep deprivation, sleep disorders such as sleep apnea, and certain medications. Regular physical exercise is one of the most effective ways to increase the proportion of deep sleep. Consistently poor quality or insufficient deep sleep is associated with impaired immune function, reduced physical recovery, cognitive difficulties, and long-term health risks.