Sleep is a naturally recurring state of mind and body, characterized by altered consciousness relatively inhibited sensory activity, reduced muscle activity and inhibition of nearly all voluntary muscles during rapid eye movement sleep, and reduced interactions with surroundings. It is distinguished from wakefulness by a decreased ability to react to stimuli.
Sleep occurs in repeating periods, in which the body alternates between two distinct modes: REM sleep and non-REM sleep. A well-known feature of sleep is the dream, an experience typically recounted in narrative form, which resembles waking life while in progress. During sleep, most of the body’s systems are in an anabolic state, helping to restore the immune, nervous, skeletal, and muscular systems; these are vital processes that maintain mood, memory, and cognitive function, and play a large role in the function of the endocrine and immune systems. The internal circadian clock promotes sleep daily at night.
Humans may suffer from various sleep disorders, including dystonias such as insomnia, hypersomnia, narcolepsy, and sleep apnoea; parasomnias such as sleepwalking and Rapid eye movement sleep behaviour disorder, bruxism; and circadian rhythm sleep disorders. The advent of artificial lights has substantially altered sleep timing in industrialized countries.
The most pronounced physiological changes in sleep occur in the brain. The brain uses significantly less energy during sleep than it does when awake, especially during non-REM sleep. In areas with reduced activity, the brain restores its supply of adenosine triphosphate.
Sleep increases the sensory threshold. In other words, sleeping persons perceive fewer stimuli, but can generally still respond to loud noises and other salient sensory events.
Sleep is divided into two broad types: non-rapid eye movement sleep and rapid eye movement sleep. The sleep cycle of alternate non-rapid eye movement sleep and rapid eye movement sleep takes an average of 90 minutes, occurring 4–6 times in a good night’s sleep.
Awakening can mean the end of sleep, or simply a moment to survey the environment and readjust body position before falling back asleep.