The neuroscience of sleep is the study of the neuroscientific and physiological basis of the nature of sleep and its functions. Traditionally, sleep has been studied as part of psychology and medicine. The study of sleep from a neuroscience perspective grew to prominence with advances in technology and the proliferation of neuroscience research from the second half of the twentieth century. The importance of sleep is demonstrated by the fact that every day animals spend hours of their time in sleep, and that sleep deprivation can have disastrous effects ultimately leading to death in animals. For a phenomenon so important, the purposes and mechanisms of sleep are only partially understood, so much so that as recently as the late 1990s it was quipped: "The only known function of sleep is to cure sleepiness". However, the development of improved imaging techniques like EEG, PET and fMRI, along with faster computers have led to an increasingly greater understanding of the mechanisms underlying sleep. The fundamental questions in the neuroscientific study of sleep are:
What are the correlates of sleep i.e. what are the minimal set of events that could confirm that the organism is sleeping? How is sleep triggered and regulated by the brain and the nervous system? What happens in the brain during sleep? How can we understand sleep function based on physiological changes in the brain? What causes various sleep disorders and how can they be treated? Other areas of modern neuroscience sleep research include the evolution of sleep, sleep during development and aging, animal sleep, mechanism of effects of drugs on sleep, dreams and nightmares, and stages of arousal between sleep and wakefulness.
Introduction
Rapid eye movement sleep (REM), non-rapid eye movement sleep (NREM or non-REM), and waking represent the three major modes of consciousness, neural activity, and physiological regulation. NREM sleep itself is divided into multiple stages: N1, N2 and N3. Sleep proceeds in 90-minute cycles of REM and NREM, the order normally being N1 → N2 → N3 → N2 → REM. As humans fall asleep, body activity slows down. Body temperature, heart rate, breathing rate, and energy use all decrease. Brain waves slow down. The excitatory neurotransmitter acetylcholine becomes less available in the brain. Humans often maneuver to create a thermally friendly environment—for example, by curling up into a ball if cold. Reflexes remain fairly active. REM sleep is considered closer to wakefulness and is characterized by rapid eye movement and muscle atonia. NREM is considered to be deep sleep (the deepest part of NREM is called slow wave sleep), and is characterized by lack of prominent eye movement, or muscle paralysis. Especially during non-REM sleep, the brain uses significantly less energy during sleep than it does in waking. In areas with reduced activity, the brain restores its supply of adenosine triphosphate (ATP), the molecule used for short-term storage and transport of energy. (Since in quiet waking the brain is responsible for 20% of the body's energy use, this reduction has an independently noticeable impact on overall energy consumption.) During slow-wave sleep, humans secrete bursts of growth hormone. All sleep, even during the day, is associated with the secretion of prolactin. According to the Hobson & McCarley activation-synthesis hypothesis, proposed in 1975–1977, the alternation between REM and non-REM can be explained in terms of cycling, reciprocally influential neurotransmitter systems. Sleep timing is controlled by the circadian clock, and in humans, to some extent by willed behavior. The term circadian comes from the Latin circa, meaning "around" (or "approximately"), and diem or dies, meaning "day". The circadian clock refers to a biological mechanism that governs multiple biological processes causing them to display an endogenous, entrainable oscillation of about 24 hours. These rhythms have been widely observed in plants, animals, fungi and cyanobacteria.
Correlates of sleep
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