Sleep

Sleep is a recurring state of reduced responsiveness to the environment that can be reversed by ordinary stimulation. It is not simply a shutdown: brain activity changes in organized ways, and the brain and body continue to perform many functions. Sleep is important for learning and forming memories, and sleep loss can impair concentration and response speed. It affects many body systems, but scientists continue to investigate its full purposes and the mechanisms behind its effects.

Human sleep has two broad types: non-rapid eye movement (NREM) sleep and rapid eye movement (REM) sleep. NREM sleep is divided into three stages, from the transition into sleep to deeper sleep. Across a typical night, people cycle through NREM and REM several times; REM periods generally become longer later in the sleep period. The stages have different patterns of brain activity and bodily changes. Most vivid dreaming is associated with REM sleep, although dreams can also occur in NREM sleep.

Sleep timing reflects the interaction of at least two internal processes. The circadian system coordinates rhythms across roughly a day and helps align sleep and wakefulness with the light–dark cycle. In the brain, the suprachiasmatic nucleus in the hypothalamus receives light-related information from the eyes and helps regulate these rhythms. A separate homeostatic sleep drive grows with time spent awake and is reduced during sleep. Light exposure, age, health, stress, medicines and work schedules can all affect sleep timing or quality.

Sleep is linked to learning, memory, mood, metabolism and immune function. Insufficient or poor-quality sleep can interfere with attention, decision-making and daily activities; chronic deficiency is associated with a range of health risks, although associations do not by themselves establish that sleep loss is the sole cause of a condition. Sleep needs vary by age and among individuals. A joint American Academy of Sleep Medicine and Sleep Research Society consensus statement recommends that adults aged 18–60 sleep at least seven hours regularly to promote health; it also emphasizes that duration is only one aspect of healthy sleep, alongside quality, timing and regularity.

Sleep deficiency is broader than simply getting too few hours. It can also involve sleeping at the wrong time of day, poor-quality sleep, missing needed sleep stages or a sleep disorder that disrupts rest. Its effects can include impaired learning, focus and reactions, with consequences for work, school, driving and health. Sleep research continues to examine how sleep is controlled and how its stages contribute to cognition and bodily function; despite substantial evidence about its importance, the full biological purpose of sleep is not settled.