Non-REM sleep, abbreviated as NREM sleep, refers to all stages of sleep that are not REM (Rapid Eye Movement) sleep. It comprises the majority of total sleep time and is divided into three progressively deeper stages: N1, N2, and N3. Together, NREM and REM sleep alternate in cycles of approximately 90 minutes throughout the night, with NREM sleep predominating in the earlier cycles and REM sleep becoming more prominent toward morning. Stage N1 is the lightest stage of sleep, representing the transition from wakefulness to sleep. It typically lasts only a few minutes, during which brain waves begin to slow, muscle activity decreases, and the person can be easily awakened. Brief muscle twitches (hypnic jerks) are common during this stage. Stage N2 is a slightly deeper stage of sleep during which the brain produces characteristic bursts of brain wave activity known as sleep spindles and K-complexes. Body temperature drops, heart rate slows, and the person becomes less aware of their surroundings. N2 sleep makes up the largest proportion of total sleep time, typically around 50 percent. Stage N3, also known as deep sleep or slow-wave sleep, is the deepest and most restorative stage of NREM sleep. It is characterized by slow, high-amplitude delta waves. During N3, the body carries out essential repair and growth processes, the immune system is strengthened, and declarative memories are consolidated. This stage is the most difficult to wake from. NREM sleep, and deep sleep in particular, is critical for physical restoration, memory consolidation, hormonal regulation, and immune function. Disruptions to NREM sleep — from sleep disorders, medications, or lifestyle factors — can significantly impair these vital processes.