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Optimal sleep amount An American study linked life spans with sleep amount and found that those who sleep 4 to 7 hours live the longest, with those sleeping less than 4 or more than 9 hours living shorter lives•. The National Sleep Foundation however maintains that 8 hours of sleep is optimal, claiming improved performance in tests, reduced risk of accidents and a better immune system. It is important to mention this does not apply to children and adolescents, particularly children who require as much as 13 hours a night. Sleep physiology The cycle between sleep and wakefulness involves different stages of sleep. Currently, scientists divide sleep into two general types: REM (Rapid Eye Movement) and NREM (non-REM). REM sleep is characterized by desynchronization of the electroencephalograph (EEG), loss of skeletal muscle tone, and sympathetic nervous system activity; whereas non-rapid eye movement sleep is characterized by parasympathetic nervous system activity (Legramante & Galante, 2005). Stages of sleep
Regulation of sleep The cycle of sleep and wakefulness is regulated by the brain stem, thalamus, external stimuli, and various hormones produced by the hypothalamus. Some neurohormones and neurotransmitters are highly correlated with sleep and wake states. For example, melatonin levels are highest during the night, and this hormone appears to promote sleep. Adenosine, a nucleoside involved in generating energy for biochemical processes, gradually accumulates in the human brain during wakefulness though decreases during sleep. Researchers believe that its accumulation during the day encourages sleep. The stimulant properties of caffeine are attributed to its negating the effects of adenosine. However the role of adenosine is far from proven, as mice lacking adenosine receptors display normal sleep patterns and normal responses to sleep deprivation. The suprachiasmatic nucleus (SCN) of the hypothalamus plays an important role and also generates its own rhythm in isolation. In the presence of light it sends messages to the pineal gland that instruct it to cease secreting melatonin. Thus, three processes, each influenced by hormonal, neurological, and environmental factors, underlie sleep regulation: The interrelationships and relative importance of each process and system remain uncertain. Theories regarding the function of sleep These several theories are not mutually exclusive; each may contain truths that will be validated in the future. Despite decades of intense research, scientists still have only clues to sleep function. With the recent demonstration that sleep is phylogenetically ancient (Shaw et al Science 2000), Hendricks et al Neuron 2000), the focus for understanding the purpose of sleep shifts from humans and other mammals to simple animals that predated the emergence of arthropoda and chordata phyla. Therefore, some of the sleep features that are unique to mammals (e.g.REM sleep and thermoregulation) are unlikely to have played a role in the evolution of a sleep like state in the premordial metazoan. Dreaming Dreaming involves an involuntary conjuring up of images in a sequence in which the sleeper/dreamer is usually more a participant than an observer. Most scientists agree that dreaming is stimulated by the pons and occurs during the REM phase of sleep. Many functions have been hypothesized for dreaming. Freud postulated that dreams are the symbolic expression of frustrated desires that had been relegated to the subconscious, and used dream interpretation in the form of psychoanalysis he pioneered. Scientists today have generally become more skeptical about details of Freudian interpretation, and place more emphasis on dreaming as a requirement for organization and consolidation of recent memory and experience. Another hypothesis is that dreaming allows an animal to play out scenarios that may help the animal avoid dangers when awake. For example, a rabbit might dream about being cornered by a fox and may play out different scenarios that might increase its chances of survival should it come across a fox in reality. Hobson and McCarley's activation synthesis theory proposes that dreams are caused by random firings of neurons in the cerebral cortex during the REM period. According to the theory, the forebrain then creates a story in an attempt to reconcile and make sense of the nonsensical sensory information presented to it, hence the odd nature of many dreams. Anthropology of sleep Recent 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 pre-light cultures have more broken up sleep patterns. For example, people might go to sleep far more quickly after the sun sets, but would 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 sleep in these societies is most often split into two main periods, the first characterised primarily by "slow sleep" and the second by REM sleep. This is called segmented sleep, which led to expressions such as "first sleep," "watch," and "second sleep" which appear in literature from all over the world in pre-industrial societies. Some societies display a fragmented sleep pattern in which people sleep at all times of the day and night for shorter periods. For example, many Mediterranean societies have a siesta, in which people sleep for a period in the afternoon. In many nomadic or hunter-gatherer societies people will sleep off and on 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-nineteenth 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, and sleep much later, although this is not always true. In some societies people generally sleep with at least one other person, often many, or with animals. In others people rarely sleep with anyone but a most intimate relation such as a spouse. In almost all societies sleeping partners are strongly regulated by social standards. For example, people might only sleep with their immediate family, extended family, spouses, with their children, with children of a certain age, children of specific gender, peers of a certain gender, friends, peers of equal social rank, or with no one at all. Sleep may be an actively social time depending on the sleep groupings, with no constraints on noise or activity. 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 head rests, 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. Cattle, horses, and sheep are unique in that they can sleep while standing, though none may experience REM sleep in this position, for REM sleep causes partial muscle paralysis. For REM sleep to take place, the animals must lie down. Conversely, birds may have periods of REM sleep while perched. Some breeds of dogs usually sleep throughout the day like cats, while other breeds have only one daily sleep session. While dreaming, dogs may make a quiet barking sound while both cats and dogs may make running motions with their legs. Sleeping routines for young children Children ages three to five, on average, should be sleeping 11-13 hours a night to promote optimal development and growth. When a child is getting enough rest, they are more likely to: The National Sleep Foundation recommends the main tip for promoting healthy sleeping habits in children is to follow a nightly routine. Achieving a nightly schedule may be difficult at first, especially if a child is not accustomed to a routine. A bedtime ritual that a child is familiar with will: Clinical patterns in aging Changes in the amount of REM sleep and latency of REM (time from sleep onset to the first REM sleep period) in the elderly are controversial. While some studies report a reduction in the percentage of REM, others report few or no changes in REM with aging. Feinberg reported a shorter REM latency and longer duration of REM sleep in the first sleep cycle. Studies looking at gender differences have shown that despite the tendency for women to complain about their sleep more often than men, older women sleep somewhat better than older men. Sleep and breathing Breathing patterns change significantly from wakefulness at sleep onset and during different sleep stages. Pathologic breathing during sleep results in increased morbidity and mortality. See Sleep and breathing for details. See also Footnotes | |||||||||||
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