Sleep can follow a physiological or behavioral definition. In the physiological sense, sleep is a state characterized by reversible unconsciousness, special brainwave patterns, sporadic eye movement, loss of muscle tone (possibly with some exceptions; see below regarding the sleep of birds and of aquatic mammals), and a compensatory increase following deprivation of the state, this last known as sleep homeostasis (i.e., the longer a waking state lasts, the greater the intensity and duration of the sleep state thereafter). Rechtschaffen A, Kales A (1968): A Manual of Standardised Terminology, Techniques and Scoring System of Sleep Stages of Human Subjects; Public Health Service, Government Printing Office, Washington. Iber C, Ancoli-Israel S, Chesson A Jr, Quan S (2007): AASM Manual for the Scoring of Sleep and Associated Events: Rules, Terminology and Technical Specification; American Association of Sleep Medicine. In the behavioral sense, sleep is characterized by minimal movement, non-responsiveness to external stimuli (i.e. increased sensory threshold), the adoption of a typical posture, and the occupation of a sheltered site, all of which is usually repeated on a 24-hour basis. The physiological definition applies well to birds and mammals, but in other animals (whose brain is not as complex), the behavioral definition is more often used. In very simple animals, behavioral definitions of sleep are the only ones possible, and even then the behavioral repertoire of the animal may not be extensive enough to allow distinction between sleep and wakefulness. Sleep is quickly reversible, as opposed to hibernation or coma, and sleep deprivation is followed by longer or deeper rebound sleep. Sleep in fish is subject of current scientific research.Reebs, S. (1992) Sleep, inactivity and circadian rhythms in fish. pp. 127–135 in: Ali, M.A. (ed.), Rhythms in Fish, New York: Plenum Press. Typically fish exhibit periods of inactivity but show no significant reactions to deprivation of this condition. Some species that always live in shoals or that swim continuously (because of a need for ram ventilation of the gills, for example) are suspected never to sleep. There is also doubt about certain blind species that live in caves. Parzefall, J. (1993): Behavioural ecology of cave-dwelling fish; pp. 573–606 in: Pitcher, T.J. (ed.), The Behaviour of Teleost Fish; London: Chapman Hall. Other fish seem to sleep, however. For example, zebrafish, and criticism and rebuttal, at PLoS Biology tilapia, tench, brown bullhead, and swell shark become motionless and unresponsive at night (or by day, in the case of the swell shark); Spanish hogfish and blue-headed wrasse can even be lifted by hand all the way to the surface without evoking a response.Tauber, E.S., 1974, The phylogeny of sleep, pp. 133–172 in: Advances in sleep research, vol. 1 (E.D. Weitzman, ed.), Spectrum Publications, New York. A 1961 observational study of approximately 200 species in European public aquaria reported many cases of apparent sleep. On the other hand, sleep patterns are easily disrupted and may even disappear during periods of migration, spawning, and parental care.
Sleep can follow a physiological or behavioral definition. In the physiological sense, sleep is a state characterized by reversible unconsciousness, special brainwave patterns, sporadic eye movement, loss of muscle tone (possibly with some exceptions; see below regarding the sleep of birds and of aquatic mammals), and a compensatory increase following deprivation of the state, this last known as sleep homeostasis (i.e., the longer a waking state lasts, the greater the intensity and duration of the sleep state thereafter). In the behavioral sense, sleep is characterized by minimal movement, non-responsiveness to external stimuli (i.e. increased sensory threshold), the adoption of a typical posture, and the occupation of a sheltered site, all of which is usually repeated on a 24-hour basis. The physiological definition applies well to birds and mammals, but in other animals (whose brain is not as complex), the behavioral definition is more often used. In very simple animals, behavioral definitions of sleep are the only ones possible, and even then the behavioral repertoire of the animal may not be extensive enough to allow distinction between sleep and wakefulness. Sleep is quickly reversible, as opposed to hibernation or coma, and sleep deprivation is followed by longer or deeper rebound sleep. Sleep in fish is subject of current scientific research.Reebs, S. (1992) Sleep, inactivity and circadian rhythms in fish. pp. 127–135 in: Ali, M.A. (ed.), Rhythms in Fish, New York: Plenum Press. Typically fish exhibit periods of inactivity but show no significant reactions to deprivation of this condition. Some species that always live in shoals or that swim continuously (because of a need for ram ventilation of the gills, for example) are suspected never to sleep. There is also doubt about certain blind species that live in caves. Other fish seem to sleep, however. For example, zebrafish, ; reply: tilapia, tench, brown bullhead, and swell shark become motionless and unresponsive at night (or by day, in the case of the swell shark); Spanish hogfish and blue-headed wrasse can even be lifted by hand all the way to the surface without evoking a response.Tauber, E.S., 1974, The phylogeny of sleep, pp. 133–172 in: Advances in sleep research, vol. 1 (E.D. Weitzman, ed.), Spectrum Publications, New York. A 1961 observational study of approximately 200 species in European public aquaria reported many cases of apparent sleep. On the other hand, sleep patterns are easily disrupted and may even disappear during periods of migration, spawning, and parental care.
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Rechtschaffen A, Kales A (1968): A Manual of Standardised Terminology, Techniques and Scoring System of Sleep Stages of Human Subjects; Public Health Service, Government Printing Office, Washington. Iber C, Ancoli-Israel S, Chesson A Jr, Quan S (2007): AASM Manual for the Scoring of Sleep and Associated Events: Rules, Terminology and Technical Specification; American Association of Sleep Medicine.
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Parzefall, J. (1993): Behavioural ecology of cave-dwelling fish; pp. 573–606 in: Pitcher, T.J. (ed.), The Behaviour of Teleost Fish; London: Chapman
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Hall.
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and criticism and rebuttal, at PLoS Biology
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