Sleep is the body's natural rest cycle, initiated by a complex interaction of hormones that respond to internal and external signals. Approximately 80% of sleep is dreamless and is referred to as non-rapid eye movement (NREM) sleep. During this phase, breathing and heart rate are slow and steady, blood pressure is reduced, and the sleeper remains relatively motionless. NREM sleep is categorized into four stages of increasing depth: Stage 1 is a transitional phase between wakefulness and sleep; Stage 2 involves a noticeable reduction in heart rate and breathing, comprising about half of total sleep time; and Stages 3 and 4, also known as Delta sleep, are characterized by very slow heart rate and breathing. Stage 4 seamlessly transitions into rapid eye movement (REM) sleep, also called Stage 5. REM sleep occurs three to five times a night, with intervals ranging from one to two hours and varying in duration. This stage features irregular breathing, fluctuating heart rate, and occasional muscle twitches. Most adults require approximately eight hours of sleep on a consistent schedule to function optimally, though individual needs may vary. It is often suggested that men need an hour less sleep than women. Children, especially teenagers, frequently need nine to ten hours for best performance.
