Sleep

Sleep is a naturally recurring state of rest for the body and mind, characterized by decreased consciousness, reduced sensory activity, and temporary muscle atonia. It is essential for health and is observed in humans and many animals. Sleep is organized into two main types: non-rapid eye movement (NREM) and rapid eye movement (REM) sleep, which cycle through the night. NREM includes three stages, from light sleep to deep slow-wave sleep, while REM sleep features vivid dreaming and stimulation of brain activity similar to wakefulness, with near-total muscle relaxation. A typical adult experiences several 90-minute sleep cycles per night, totaling about 7 to 9 hours under ideal conditions.

Sleep is regulated by homeostatic processes and the circadian system, which synchronizes sleep timing with the 24-hour day. The circadian rhythm is driven by the brain’s suprachiasmatic nucleus and is influenced by light exposure; the pineal gland releases melatonin to promote sleepiness at night. Sleep serves multiple functions, including restoration of tissues and metabolic processes, consolidation of memories and learning, and clearance of metabolic waste from the brain via the glymphatic system.

Disrupted or insufficient sleep is associated with daytime impairment, mood disturbances, and increased risk for various chronic conditions. Common sleep disorders include insomnia (difficulty initiating or maintaining sleep), obstructive sleep apnea (breathing interruptions during sleep), narcolepsy (uncontrollable sleep attacks), restless legs syndrome, and parasomnias such as sleepwalking or night terrors.

Guidelines vary by age, but healthy adults are generally advised to aim for about 7 to 9 hours per night, with greater needs for children and adolescents. Habits that support sleep include consistent schedules, a dark and quiet environment, limiting caffeine and heavy meals before bedtime, and regular physical activity.