Sleep
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In the West, a bed, blankets, pillows, and pajamas are all traditionally associated with sleep. |
Sleep is the regular state of natural rest observed in all
mammals,
birds, and
fish. Sleep is not actually "
unconsciousness," but rather, 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, sleep is necessary for life. The capability for arousal from sleep is a protective mechanism and also necessary for health and survival.
The cycle between deepest sleep and wakefullness involves different stages of sleep. The activities of the body are different at the different stages. Currently, scientists divide sleep into two general types: REM and NREM. REM sleep is characterized by desynchronization of EEG and loss of skeletal muscle tone. Whereas blood pressure and heart rate tend to decrease during non-rapid eye movement sleep, periods of faster heartbeat and higher blood pressure characterize REM periods.(Legramante JM. Galante A. Sleep and hypertension: a challenge for the autonomic regulation of the cardiovascular system.[comment]. [Review] [25 refs] [Comment. Editorial. Review] Circulation. 112(6):786-8, 2005 Aug 9)
Stages of sleep
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Stage 1 Sleep. EEG highlighted by red box. |
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Stage 2 Sleep. EEG highlighted by red box. Sleep spindles highlighted by red line. |
Studies of human sleep have established five well-defined stages, according to
electroencephalographic (EEG) recordings and
polysomnography:
*
Non-REM sleep (
NREM), 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 can be associated with so-called
hypnagogic hallucinations. In 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" (12â€"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,
bedwetting, and
sleepwalking occur.
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Stage 4 Sleep. EEG highlighted by red box. |
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REM Sleep. EEG highlighted by red box. Eye movements highlighted by red line. |
*
Rapid eye movement sleep (
REM), 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 below). 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.
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 but 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 adenosine1 receptors display normal sleep patterns and normal responses to sleep deprivation.
The
suprachiasmatic nucleus (SCN) of the hypothalamus plays an important role in the regulation of
circadian rhythms. The SCN is influenced by external light 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.
*
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. Sleep deprivation, in some studies, leads to decrements in immune function and can alter
metabolism under extreme, extended sleep deprivation paradigms. 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. Overall there is currently only scant evidence in support of the restorative hypothesis.
**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 behavioral 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. Mere
rote memorization is retained similarly well 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 maximizes 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. The fact is that sleep is itself a "drive" in that animals will alter their behaviors in order to obtain sleep: sleep is not simply a passive consequence of removing the animal from the environment. Therefore, circadian regulation is more than sufficient to explain periods of activity and quiescence that are adaptive to an organism but the more peculiar specializatons 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.
The demonstrably necessary phenomenon of dreaming would suffice to prove the importance of sleep to humans, and perhaps to other animals as well. 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 theory 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. Critics of this theory, however, resent the implicit notion that reason and emotion are necessarily absent from dreams, and point out that not all dreams are entirely random; some individuals and cultures value their dreams and see dreaming as a meaningful, important part of life. Also, this theory has difficulty accounting for the demonstrable phenomenon of
recurring dreams.
Recent research suggests that sleep patterns vary significantly across cultures.
[Toward a Comparative Developmental Ecology of Human Sleep] 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 and cats sometimes make a quiet barking sound or make running motions with their legs.
* Gilmartin GS, Thomas RJ. Mechanisms of arousal from sleep and their consequences. [Review] [46 refs] [Journal Article. Review] Current Opinion in Pulmonary Medicine. 10(6):468-74, 2004 Nov.
* Gottlieb, D. J., Punjabi, N. M., Newman, A. B., Resnick, H. E., Redline, S., Baldwin, C. M., et al. (2005). Association of sleep time with diabetes mellitus and impaired glucose tolerance. Arch Intern Med, 165(8), 863-867.
* Morrissey, M. J., Duntley, S. P., Anch, A. M., & Nonneman, R. (2004). Active sleep and its role in the prevention of apoptosis in the developing brain. Medical Hypotheses, 62(6), 876-879.
* Marks, G. A., Shaffery, J. P., Oksenberg, A., Speciale, S. G., & Roffwarg, H. P. (1995). A functional role for rem sleep in brain maturation. Behav Brain Res, 69(1-2), 1-11.
* Mirmiran, M., Scholtens, J., van de Poll, N. E., Uylings, H. B., van der Gugten, J., & Boer, G. J. (1983). Effects of experimental suppression of active (rem) sleep during early development upon adult brain and behavior in the rat. Brain Res, 283(2-3), 277-286.
* Zhang, J. (2004).
Memory process and the function of sleep. Journal of Theoretics.Vol 6-6.
Foldvary-Schaefer N. Grigg-Damberger M. Sleep and epilepsy: what we know, don't know, and need to know. [Review] [242 refs] [Historical Article. Journal Article. Review] Journal of Clinical Neurophysiology. 23(1):4-20, 2006 Feb.
*
American Academy of Sleep Medicine*
Everything you wanted to know about sleep and sleep disorders*
Sleep Research Society*
National Sleep Foundation*
National Center on Sleep Disorders Research*
How to Get Great Sleep*
Down for the Count *
Tips for Sleeping*
Sleep primer from the journal
Nature*
Sleep Apnea, Insomnia, Narcolepsy Foundational Resources*
Sleeping Advice *
Sleep News*
TalkAboutSleep.com Provides information and resources on all 80+ sleep disorders including: Sleep Apnea, Insomnia, Narcolepsy, Fibromyalgia, etc.
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The Basics of Sleep Behavior *
Getting the Sleep You Need: Sleep Stages, Sleep Tips and Aids*
Sleep: Understanding its importance (article from thedoctorslounge.net)
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Going without sleep Scientific American response to question from the public
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Sleep and Obesity*
Neurobehavioral Consequenses of Sleep Dysfunction*
Determinants of Obesity and Metabolic Dysfunction in Adolescents*
Metabolic Dysfunction Associated with Sleep-disordered Breathing Increases the Risk of Cardiovascular Morbidity and Mortality*
Improvement of Metabolic Function in Sleep Apnea*
Slumber's Unexplored Landscape*
C.M. Worthman and M. Melby. Toward a comparative developmental ecology of human sleep.*
The Roles Of Rest And Sleep In Supplying Body Needs
