Sleep is a natural, recurring state of rest that is essential for the health and functioning of all living organisms. During sleep, the body and mind undergo vital restorative processes that are critical for physical health, cognitive function, emotional regulation, and overall wellbeing. Sleep is not a passive or uniform state. It is an active biological process consisting of multiple distinct stages that cycle repeatedly throughout the night. These stages include light sleep, deep sleep (slow-wave sleep), and REM (rapid eye movement) sleep. Each stage serves different restorative functions: deep sleep is associated with physical repair, immune strengthening, and memory consolidation, while REM sleep is closely linked to emotional processing, learning, and dreaming. The amount of sleep required varies across the lifespan. Adults generally need 7 to 9 hours per night, while children and teenagers require more. Sleep is regulated by two key biological processes: the circadian rhythm, an internal 24-hour clock that determines timing of sleepiness and alertness, and sleep pressure (homeostatic sleep drive), which builds gradually during wakefulness and is relieved by sleep. Chronic sleep deprivation has serious health consequences. It is associated with increased risk of obesity, cardiovascular disease, type 2 diabetes, impaired immune function, depression, anxiety, and reduced life expectancy. Conversely, adequate sleep supports healthy metabolism, immune function, cognitive performance, mood, and physical recovery. Despite its fundamental importance, sleep is often undervalued in modern society. Prioritizing sleep as a core health behavior — alongside nutrition and exercise — is increasingly recognized as essential for long-term health and quality of life.