The sleep phase refers to the specific period within the sleep-wake cycle during which a person is asleep. Human sleep consists of multiple sleep phases, which repeat in cycles of approximately 90 minutes throughout the night. Each phase has distinct characteristics and plays a different role in physical and mental restoration. Sleep is broadly divided into two main phases: non-REM (NREM) sleep and REM (rapid eye movement) sleep. Non-REM sleep is further divided into three stages. Stage 1 is a light transitional phase between wakefulness and sleep. Stage 2 is characterized by slowing brain waves, reduced heart rate, and a drop in body temperature — this stage makes up the largest proportion of total sleep time. Stage 3, often called deep sleep or slow-wave sleep, is the most physically restorative phase, during which the body repairs tissues, strengthens the immune system, and consolidates memories. REM sleep follows non-REM sleep and is characterized by rapid eye movements, vivid dreaming, and brain activity similar to wakefulness. REM sleep plays a crucial role in emotional regulation, memory consolidation, and cognitive function. The proportion of time spent in each sleep phase changes across the night. Deep sleep is more prevalent in the early sleep cycles, while REM sleep becomes more prominent in the later cycles closer to morning. Disruptions to sleep, whether from external factors or sleep disorders, can reduce the time spent in restorative deep and REM sleep phases, leading to daytime fatigue and impaired functioning.