Changes

'''Sleep''' is a naturally recurring [[state]] of [[relatively]] suspended sensory and motor [[activity]], characterized by total or partial [[unconsciousness]] and the inactivity of nearly all voluntary muscles.[1] It is distinguished from quiet [[wakefulness]] by a decreased ability to react to stimuli, and it is more easily reversible than [[hibernation]] or [[coma]]. It is observed in all mammals, all birds, and many reptiles, amphibians, and fish. In humans, other mammals, and a substantial majority of other animals that have been [[studied]] (such as some species of fish, birds, ants, and fruit flies), regular sleep is [[essential]] for survival.

'''Sleep''' is a naturally recurring [[state]] of [[relatively]] suspended sensory and motor [[activity]], characterized by total or partial [[unconsciousness]] and the inactivity of nearly all voluntary muscles.[1] It is distinguished from quiet [[wakefulness]] by a decreased ability to react to stimuli, and it is more easily reversible than [[hibernation]] or [[coma]]. It is observed in all mammals, all birds, and many reptiles, amphibians, and fish. In humans, other mammals, and a substantial majority of other animals that have been [[studied]] (such as some species of fish, birds, ants, and fruit flies), regular sleep is [[essential]] for survival.

==Anthropology of sleep==

==Anthropology of sleep==

[[Research]] suggests that sleep [[patterns]] vary significantly across [[cultures]].  The most striking [[differences]] are between [[societies]] that have plentiful sources of artificial [[light]] and ones that do not.  The primary difference appears to be that prelight cultures have more broken-up sleep patterns.  For example, people might go to sleep far sooner after the sun sets, but then wake up several times throughout the night, punctuating their sleep with periods of [[wakefulness]], perhaps lasting several hours.  The [[boundaries]] between sleeping and waking are blurred in these societies. Some observers believe that nighttime sleep in these societies is most often split into two main periods, the first characterised primarily by deep sleep and the second by [http://en.wikipedia.org/wiki/Sleep#REM_sleep REM sleep]. This [http://en.wikipedia.org/wiki/Segmented_sleep segmented sleep] has led to [[expressions]] such as "first sleep," "watch," and "second sleep," which appear in [[literature]] from preindustrial societies all over the world.

[[Research]] suggests that sleep [[patterns]] vary significantly across [[cultures]].  The most striking [[differences]] are between [[societies]] that have plentiful sources of artificial [[light]] and ones that do not.  The primary difference appears to be that prelight cultures have more broken-up sleep patterns.  For example, people might go to sleep far sooner after the sun sets, but then wake up several times throughout the night, punctuating their sleep with periods of [[wakefulness]], perhaps lasting several hours.  The [[Liminality|boundaries]] between sleeping and waking are blurred in these societies. Some observers believe that nighttime sleep in these societies is most often split into two main periods, the first characterised primarily by deep sleep and the second by [http://en.wikipedia.org/wiki/Sleep#REM_sleep REM sleep]. This [http://en.wikipedia.org/wiki/Segmented_sleep segmented sleep] has led to [[expressions]] such as "first sleep," "watch," and "second sleep," which appear in [[literature]] from preindustrial societies all over the world.

Some societies display a fragmented sleep pattern in which people sleep at all times of the day and night for shorter periods. In many [http://en.wikipedia.org/wiki/Nomadic nomadic] or [http://en.wikipedia.org/wiki/Hunter-gatherer hunter-gatherer] societies, people will sleep on and off throughout the day or night depending on what is happening.  Plentiful artificial [[light]] has been available in the industrialised West since at least the mid-[http://en.wikipedia.org/wiki/19th_century 19th century], and sleep patterns have changed significantly everywhere that lighting has been introduced.  In general, people sleep in a more concentrated burst through the night, going to sleep much later, although this is not always true.

Some societies display a fragmented sleep pattern in which people sleep at all times of the day and night for shorter periods. In many [http://en.wikipedia.org/wiki/Nomadic nomadic] or [http://en.wikipedia.org/wiki/Hunter-gatherer hunter-gatherer] societies, people will sleep on and off throughout the day or night depending on what is happening.  Plentiful artificial [[light]] has been available in the industrialised West since at least the mid-[http://en.wikipedia.org/wiki/19th_century 19th century], and sleep patterns have changed significantly everywhere that lighting has been introduced.  In general, people sleep in a more concentrated burst through the night, going to sleep much later, although this is not always true.

People sleep in a variety of locations. Some sleep directly on the ground; others on a skin or blanket; others sleep on platforms or beds. Some sleep with blankets, some with pillows, some with simple headrests, some with no head support. These [[choices]] are shaped by a variety of factors, such as climate, protection from predators, housing type, [[technology]], and the incidence of pests.[56]

People sleep in a variety of locations. Some sleep directly on the ground; others on a skin or blanket; others sleep on platforms or beds. Some sleep with blankets, some with pillows, some with simple headrests, some with no head support. These [[choices]] are shaped by a variety of factors, such as climate, protection from predators, housing type, [[technology]], and the incidence of pests.[56]

==External links==

==External links==

* [http://healthysleep.med.harvard.edu/ Healthy Sleep] from the Division of Sleep Medicine at [http://en.wikipedia.org/wiki/Harvard_Medical_School Harvard Medical School] and [http://en.wikipedia.org/wiki/WGBH-TV WGBH] Educational Foundation

* [http://healthysleep.med.harvard.edu/ Healthy Sleep] from the Division of Sleep Medicine at [http://en.wikipedia.org/wiki/Harvard_Medical_School Harvard Medical School] and [http://en.wikipedia.org/wiki/WGBH-TV WGBH] Educational Foundation