Sleeping

Sleep is a recurring, reversible state of reduced responsiveness and diminished voluntary movement, characterized by specific physiological patterns. It alternates with wakefulness in a roughly 24-hour cycle governed by the circadian system. Sleep is essential for physical and cognitive health and is observed across many animals.

Sleep consists of non-REM sleep, subdivided N1, N2, N3 (slow-wave sleep), and REM sleep. In a typical night, individuals cycle through these stages approximately every 90 minutes, with REM periods increasing toward the morning. N3 is deep sleep associated with restorative processes; REM features rapid eye movements and more brain activity similar to wakefulness, with skeletal muscle atonia.

The circadian rhythm is regulated by the suprachiasmatic nucleus in the hypothalamus, influenced by light exposure and melatonin production. Sleep duration and timing vary by age and individual needs; inadequate sleep creates a sleep debt that can accumulate.

Functions include memory consolidation and learning, metabolic regulation, immune support, and brain waste clearance via the glymphatic system. Chronic insufficient sleep is linked to obesity, type 2 diabetes, cardiovascular disease, and mental health disorders.

Common sleep disorders include insomnia (difficulties initiating or maintaining sleep), obstructive sleep apnea (repeated airway obstruction during sleep), restless legs syndrome, and narcolepsy. Diagnosis often uses polysomnography or actigraphy. Treatments encompass cognitive-behavioral therapy for insomnia, devices or surgery for sleep apnea, and lifestyle adjustments and, when appropriate, medications. Good sleep hygiene—regular schedule, comfortable environment, limited caffeine and screen exposure—supports healthy sleep.