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Sleep is a naturally recurring state of mind characterized by altered consciousness, relatively inhibited sensory activity, inhibition of nearly all voluntary muscles, and reduced interactions with surroundings.〔''Macmillan Dictionary for Students'' Macmillan, Pan Ltd. (1981), p. 936. Retrieved 1 October 2009.〕 It is distinguished from wakefulness by a decreased ability to react to stimuli, but is more easily reversed than the state of hibernation or of being comatose. Mammalian sleep occurs in repeating periods, in which the body alternates between two highly distinct modes known as non-REM and REM sleep. REM stands for "rapid eye movement" but involves many other aspects including virtual paralysis of the body. During sleep, most systems in an animal are in an anabolic state, building up the immune, nervous, skeletal, and muscular systems. Sleep in non-human animals is observed in mammals, birds, reptiles, amphibians, and fish, and, in some form, in insects and even in simpler animals such as nematodes. The internal circadian clock promotes sleep daily at night in diurnal species (such as humans) and in the day in nocturnal organisms (such as rodents). However, sleep patterns vary widely among animals and among different individual humans. Industrialization and artificial light have substantially altered human sleep habits in the last 100 years. The diverse purposes and mechanisms of sleep are the subject of substantial ongoing research. Sleep seems to assist animals with improvements in the body and mind. A well-known feature of sleep in humans is the dream, an experience typically recounted in narrative form, which resembles waking life while in progress, but which usually can later be distinguished as fantasy. Humans may suffer from a number of sleep disorders. These include dyssomnias (such as insomnia, hypersomnia, and sleep apnea), parasomnias (such as sleepwalking and REM behavior disorder), bruxism, and the circadian rhythm sleep disorders. == Physiology == In mammals and birds, sleep is divided into two broad types: rapid eye movement (REM sleep) and non-rapid eye movement (NREM or non-REM sleep). Each type has a distinct set of physiological and neurological features associated with it. REM sleep is associated with dreaming, desynchronized and faster brain waves, loss of muscle tone, and suspension of homeostasis.〔(【引用サイトリンク】title=Brain Basics: Understanding Sleep )〕 REM and non-REM sleep are so different that physiologists classify them as distinct behavioral states. In this view, REM, non-REM, and waking represent the three major modes of consciousness, neural activity, and physiological regulation.〔 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.〔J. Alan Hobson & Robert W. McCarley, "The Brain as a Dream-State Generator: An Activation-Synthesis Hypothesis of the Dream Process", ''American Journal of Psychiatry'' 134.12, December 1977.〕 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.〔Brown ''et al.'' (2012), "Control of Sleep and Wakefulness", p. 1118–1119. "Compared with wakefulness, sleep reduces brain energy demands, as suggested by the 44% reduction in the cerebral metabolic rate (CMR) of glucose (791) and a 25% reduction in the CMR of O2 (774) during sleep."〕 (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.)〔Jerome M. Siegel (2008), "Do all animals sleep?", ''Trends in Neurosciences'' 31.4.〕 During slow-wave sleep, humans secrete bursts of growth hormone. All sleep, even during the day, is associated with secretion of prolactin.〔 Sleep increases an organism's sensory threshold. In other words, a sleeping creature perceives fewer stimuli. However, it can generally still respond to loud noises and other salient sensory events.〔 Key physiological indicators in sleep include EEG of brain waves, electrooculography (EOG) of eye movements, and electromyography (EMG) of skeletal muscle activity. Simultaneous collection of these measurements is called polysomnography and can be performed in a specialized sleep laboratory.〔Brown ''et al.'' (2012), "Control of Sleep and Wakefulness", p. 1087.〕 抄文引用元・出典: フリー百科事典『 ウィキペディア（Wikipedia）』 ■ウィキペディアで「Sleep」の詳細全文を読む スポンサード リンク
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