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Sleep

2007 Schools Wikipedia Selection. Related subjects: Everyday life

   Sleep is the state of natural rest observed in most mammals, birds,
   fish, as well as invertebrates such as the fruitfly Drosophila. It is
   characterized by a reduction in voluntary body movement, decreased
   reaction to external stimuli, an increased rate of anabolism (the
   synthesis of cell structures), and a decreased rate of catabolism (the
   breakdown of cell structures). In humans, mammals and many other
   animals which have been studied, such as fish, birds, mice and
   fruitflies, sleep is necessary for life. The capability for arousal
   from sleep is a protective mechanism and also necessary for health and
   survival. Sleep is not synonymous with unconsciousness.
   For many, a bed, blankets, pillows, and pajamas are all traditionally
   associated with sleep.
   Enlarge
   For many, a bed, blankets, pillows, and pajamas are all traditionally
   associated with sleep.

Optimal sleep amount

   An American study linked life spans with sleep amount and found that
   those who sleep 4 to 7 hours live longer lives, with those sleeping
   less than 4 or more than 9 hours having about a 30% increased chance of
   early death, similar to moderate obesity.. 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. However, this does not apply to children and
   adolescents, particularly young 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; where as
   non-rapid eye movement sleep is characterized by parasympathetic
   nervous system activity (Legramante & Galante, 2005).

Stages of sleep

     * Non-REM accounts for 75–80% of total sleep time:
          + Stage 1, with near-disappearance of the alpha waves seen in
            awake states, and appearance for the first time of theta
            waves. The stage is sometimes referred to as somnolence, or
            "drowsy sleep". It appears at sleep onset (as it is mostly a
            transition state into Stage 2) and is associated with the
            sudden twitches or hypnic jerks many people experience when
            falling asleep. While these are normal and of no concern, the
            hypnagogic hallucinations which some people may experience at
            this stage can be more troublesome. During this period, the
            subject loses some muscle tone, and conscious awareness of the
            external environment: Stage 1 can be thought of as a gateway
            state between wake and sleep.
          + Stage 2, with " sleep spindles" (12–16 Hz) and " K-complexes."
            The EMG lowers, and conscious awareness of the external
            environment disappears. This occupies 45–55% of total sleep.
          + Stage 3, with delta waves, also called delta rhythms (.5–4
            Hz), is considered part of slow-wave sleep ( SWS) and
            functions primarily as a transition into stage four. Overall
            it occupies 3–8% of total sleep time.
          + Stage 4 is true delta sleep. It predominates the first third
            of the night and accounts for 10–15% of total sleep time. This
            is often described as the deepest stage of sleep; it is
            exceedingly difficult to wake a subject in this state. This is
            the stage in which night terrors, bed wetting, and
            sleepwalking occur.

   Stage 4 Sleep. EEG highlighted by red box.
   Enlarge
   Stage 4 Sleep. EEG highlighted by red box.
   REM Sleep. EEG highlighted by red box. Eye movements highlighted by red
   line.
   Enlarge
   REM Sleep. EEG highlighted by red box. Eye movements highlighted by red
   line.
     * REM sleep is popularly associated with dreaming, especially
       bizarre, visual, and seemingly random dreams; however, dreams can
       also occur during sleep onset (hypnogogia) and during all stages of
       Non-REM sleep. REM sleep is predominant in the final third of a
       sleep period; its timing is linked to circadian rhythm and body
       temperature. The EEG in this period is aroused and looks similar to
       stage 1, and sometimes includes beta waves. Also known as Stage 5
       sleep.

   Sleep proceeds in cycles of NREM and REM phases. In humans, the cycle
   of REM and NREM is approximately 90 minutes. Each stage may have a
   distinct physiological function. Drugs such as alcohol and sleeping
   pills can suppress certain stages of sleep (see Sleep deprivation).
   This can result in a sleep that exhibits loss of consciousness but does
   not fulfill its physiological functions.

   Each sleep stage is not necessarily uniform. Within a given stage, a
   cyclical alternating pattern may be observed.

   Both REM sleep and NREM sleep stages 3 and 4 are homeostatically
   driven; that is, selective deprivation of each of these states
   subsequently causes a rebound in their appearance once the person is
   allowed to sleep. This finding leads to the ubiquitous assumption that
   both are essential in the sleep process and its many functions. REM
   sleep may also be driven by a circadian oscillator, as studies have
   shown that REM is temporally coupled with the circadian rhythm of
   temperature.

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 nucleotide 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:
     * A homeostatic process determined by prior sleep and wakefulness,
       determining "sleep need."
     * A circadian process determining periods of high and low sleep
       propensity, and high and low rapid eye movement (REM) sleep
       propensity.
     * An ultradian process.

   The interrelationships and relative importance of each process and
   system remain uncertain.

Theories regarding the function of sleep

     * Restorative theories of sleep describe sleep as a dynamic time of
       healing and growth for organisms. For example, during stages 3 and
       4, or slow-wave sleep, growth hormone levels increase, and changes
       in immune function occur. In some studies sleep deprivation has led
       to decrements in immune function and, under extreme, extended sleep
       deprivation regimes, altered metabolism. However, short periods of
       sleep deprivation have not been conclusively shown to significantly
       impact organ, muscular, cardiac, or other somatic function in ways
       that suggest that any of these systems are primarily influenced by
       sleep.

     * Non-REM sleep may be an anabolic state marked by physiological
       processes of growth and rejuvenation of the organism's immune,
       nervous, muscular, and skeletal systems (but see above). Sleep
       might restore neurons and increase production of brain proteins and
       certain hormones. Wakefulness may perhaps be viewed as a cyclical,
       temporary, hyperactive catabolic state during which the organism
       acquires nourishment and procreates. Also, during sleep, an
       organism is vulnerable; when awake it may perceive and avoid
       threats. Asking the question "Why do we awaken?" instead of "Why do
       we sleep?" yields a different perspective toward understanding how
       sleep and its stages contribute to a healthy organism.

     * According to the ontogenetic hypothesis of REM sleep, the activity
       occurring during neonatal REM sleep (or active sleep) seems to be
       particularly important to the developing organism (Marks et al.,
       1995). Studies investigating the effects of deprivation of active
       sleep have shown that deprivation early in life can result in
       behavioural problems, permanent sleep disruption, decreased brain
       mass (Mirmiran et al. 1983), and an abnormal amount of neuronal
       cell death (Morrissey, Duntley & Anch, 2004). Given sleep's
       heterogeneous nature, however, no single theory predominates, as it
       is difficult to describe one single "function" of sleep.

     * One process commonly believed to be highly dependent on sleep is
       memory. REM sleep appears to help with the consolidation of spatial
       and procedural memory, while slow-wave sleep helps with the
       consolidation of declarative memories. When experimental subjects
       are given academic material to learn, especially if it involves
       organized, systematic thought, their retention is markedly
       increased after a night's sleep. On the other hand, the
       effectiveness of mere rote memorization is similar with or without
       an intervening period of sleep. Some memory theorists argue that
       saving memory directly into long-term memory is a slow and error
       prone process, and propose that cerebral input is saved first in a
       temporary memory store, and then encoded and transferred into
       long-term memory during sleep. (Zhang, 2004).

     *
          + Despite an abundance of positive findings in support of these
            ideas, many sleep scientists do not believe that sleep's
            primary function is related to memory. These scientists point
            out that many of the studies cited by proponents of this
            theory are contradictory or confounded by the side-effects
            caused by the experimental manipulations. A more salient issue
            is that only a handful of studies have shown that sleep
            actually influences brain plasticity--which is the mechanism
            underlying remembering and forgetting (Benington and Frank,
            2003).

     * One view, "Preservation and Protection", is that sleep serves an
       adaptive function. It protects the individual during that portion
       of the 24-hour day in which being awake, and hence roaming around,
       would place the individual at greatest risk. Organisms don't
       require 24 hours to feed themselves and meet other necessities.
       From this perspective of adaptation, organisms are safer by staying
       out of harm's way where potentially they could be prey to other
       stronger organisms. They sleep at times that maximize their safety,
       given their physical capacities and their habitats. (Allison &
       Cicchetti, 1976; Webb, 1982). This theory, however, is not
       universally accepted. For example, if true, there would be no
       reason for the brain to disengage from the external environment as
       it does during normal sleep. A more advantageous adaptation would
       be for animals to seclude themselves but maintain quiet wakefulness
       to avoid predation. Sleep is not simply a passive consequence of
       removing the animal from the environment, but rather is itself a
       "drive": animals will alter their behaviors in order to obtain
       sleep. Therefore, circadian regulation is more than sufficient to
       explain periods of activity and quiescence that are adaptive to an
       organism, but the more peculiar specializations of sleep most
       likely reflect different and unknown functions.

   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. An
   examination of the nature of sleep and of wakefulness thus turns its
   focus to the study of the roles that proteins and enzymes play in basic
   metabolism.

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.

   Clothing worn for sleep varies across individuals and cultures. Many
   people wear pajamas, some a nightshirt, some regular (day) underwear,
   some the same clothes they wore during the day (changing clothes in the
   morning), and some sleep nude.

Sleep in animals

   Sleeping Japanese macaques.
   Enlarge
   Sleeping Japanese macaques.

   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 children are
   getting enough rest, they are more likely to:
     * Be more alert
     * Have a better memory
     * Show increased performance on tasks
     * Be less prone to behavioural problems
     * Have more mood stability

   Children ages 6 to 12 need 8-10 hours of sleep.

   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 children are familiar with will:
     * Make it easier to relax
     * Help them fall asleep on their own
     * Encourage them to sleep through the night

   Retrieved from " http://en.wikipedia.org/wiki/Sleep"
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