the study of sleep states has led to an increased knowledge about daily behavioural rhythms involving perception, emotion and thoughts. sleep is not an inactive state, but has its own unique electrical and neurochemical activity and physiology depending on the sleep state from which these measures are taken.
THEORIES OF SLEEP
Some theories of sleep are based on the knowledge that vigilance levels drop the longer one stays awake. sleep rebound effects (increases in REM and/or non-REM (NREM) sleep following sleep deprivation) occur in an attempt to make up for sleep loss and to restore vigilance.
1.Restorative Theories
these theories propose that while one is awake some substance(s) is being depleted or that cells suffer over-exertion. consequently, sleep occurs so that the substance(s) can be restored or so that cells can rest. A problem with this theory is that the "sleep factor" which is hypothetically depleted has never been identified and there is
no evidence to suggest that any cells fire less during sleep than in waking states. however Hobson notes that certain neurotransmitters (specifically norepinephrine (NE) and serotonin (5-HT)), which are at high levels during waking, are only barely active during NREM sleep and are inactive during REM sleep. As such it is possible that during this time these nerutransmitters are replenished, ultimately resulting in enhanced diurnal vigilance.
2.Homeostatic Theories
these theories focus on the regulation of energy. sleep deprivation experiments have demonstrated that rats who have not been allowed any sleep eventually lose the ability to control their body temperature or maintain their weight and eventually die (loss of control over metabolism and homestasis). therefore certain researchers have proposed that sleep may help to reduce energy disequilibrium by maintaining the efficiency of brain mechanisms that function to regulate energy.
3.Brain Plasticity Theories
these
theories focus on the role of sleep in leaning and memory. it is suggested that sleep is a good time for memory processing to occur because it is not subject to continuous input to the brain as is experienced during waking, and as such, information is not interfered with by new stimuli. some researchers, Crick & Mitcheson for example, argue that sleep is for forgetting and that useless information is discarded during sleep (especially REM). One problem with this hypothesis is that it has not been demonstrated how the brain is capable of deciding what information should be discarded and which it should keep. conversely, other researchers propose that sleep aids in the consolidation of new information by providing time to assimilate, store and catalogue the information within already existing memories in the brain. it should be noted that both types of theories have received support in the experimental literature.
4.Species Typical Programming
this is another brain plasticity theory which
assumes that many behaviours have a large genetic component. these behaviours are present at birth, but usually require adjustment or have a learned component as well, as such, sleep provides the opportunity for these behaviours to be rehearsed. One example of this type of behaviour is the predatory behaviour of the house cat. even though these animals have never had to hunt, prdatory behaviour can be experimentally induced by implanting electrodes into brain regions responsible for the activation of hunting behaviour (periaqueductal gray matter (PAQ)). moreover, when muscle paralysis (atonia) during REM is eliminated, cats appear to act out hunting behaviours in this dream state. it is therefore hypothesized that because these behaviours are rarely used they must be practiced so that they can act when the opportunity to do so arises.
5.Developmental Theories
infants and young children engage in much larger quantities of sleep than adults. as such it has been suggested that sleep may play an
important role in learning or adjusting to new environments. some researchers have proposed that much of this learning could take place during REM sleep. for example, visual and basic motor systems may develop in this way prenatally. in REM there is substantial brainstem activity which could provide a stimulus to developing cerebral circuits.
DISORDERED SLEEP
sleep disorders generally fall into one of 4 categories:
Dyssomnias
intrinsic sleep disorders include: psychophysiological insomnia, sleep state misperception, idiopathic insomnia, narcolepsy, hypersomnias, sleep apneas, periodic limb movement disorder (PLM), and restless legs syndrome.
extrinsic dysomnias include:
circadian rhythm disorders include:
parasomnias include:
References
Hobson J., (1995). Sleep. New York: Scientific American Library.
Smith C., (1998). Psychology 372 class notes. Trent University.
an essay on sleep and anxiety disorders
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