ABC ofSleep Disorders FUNCTION OF SLEEP, Exams of Abnormal Psychology

Download ABC ofSleep Disorders FUNCTION OF SLEEP and more Exams Abnormal Psychology in PDF only on Docsity! ABC ofSleep Disorders FUNCTION OF SLEEP CM Shapiro,M J Flanigan Electroencephalographs of a college student. The horizontal axis of each tracing is divided into units of one second. The top tracing is from one electrode on the scalp and the lower tracing indicates eye movements. Note the presence of slow waves in stages 2,3, and 4. Sleep comprises two distinct physiological states, as different from each other as each one is from wakefulness; they are known as rapid eye movement (REM) sleep and non-REM sleep. Non-REM sleep is made up offour stages: stage 1 is a light, drowsy phase-the transition from wakefulness to sleep; stage 2 is the first real stage of sleep, with the appearance of sleep "spindles" and "K complexes" on the electroencephalogram; and stages 3 and 4 are known collectively as "slow wave sleep," or deep sleep, because of the emergence oflow frequency, synchronised waves. REM sleep is the stage during which most dreaming occurs. As people fall asleep they progress through the non-REM stages and then about 90 minutes later they have the first episode ofREM sleep. There is a cycle of non-REM and REM sleep throughout the night, and as the night progresses the episodes ofnon-REM sleep become shorter, and those ofREM sleep longer. Most slow wave sleep occurs during the first third ofthe night, and most REM sleep during the last third. Only during the past few decades has sleep been described in electrophysiological terms. As a result there has been a vast increase in the number oftechniques that clinicians and research workers use to record and analyse the electrophysiological measurements of sleep. As knowledge about the physiology of sleep has increased, doctors have become aware of the variety ofproblems and abnormalities associated with sleep that are common among the general population, and so the discipline of sleep disorders has developed. Despite the wealth ofinformation that is accumulating about the biochemistry and physiology of sleep, its precise nature and function are not known. A number oftheories have been proposed, which include the hypotheses that sleep is needed: for consolidation ofmemory, for binocular vision, or as part of thermoregulatory evolution. In this introduction we will deal with two of the more accepted theories, and this explanation will provide both a synthesis of relevant research, and background to subsequent articles that deal with details ofthe pathology of sleep. Cycles of REM and non-REM sleep. Each cycle lasts roughly 90 minutes; slow wave sleep predominates during the first third of the night and REM sleep during the last third. Conservation ofenergy Most people's general activities increase during the day compared with the night. The concept ofhomoeostasis may be extended to explain that energy that is expended during the day must be balanced by a recuperative period. This forms the foundation ofone of the theories ofthe function of sleep-that ofconservation of energy. Expenditure ofenergy is measured mainly by the metabolic rate, which is raised during the day and reduced during the night (particularly during sleep) by between 5% and 25%. BMJ VOLUME 306 6 FEBRUARY 1993 Themtboltic rates uedt nt. andi . i. b 383 Number of hours of sleep/24 hours required by various animal species. Animals that are seldom attacked sleep a great deal; those in constant danger of attack sleep little. Oxygen consumption, heart rate, and body temperature decline during the first few hours of sleep-the time particularly associated with slow wave sleep-and it is postulated that slow wave sleep is strongly associated with conservation of energy. There is a relationship across species between metabolic rate and sleep pattern, and there is evidence that people whose metabolic rates are high during the day have more slow wave sleep and sleep longer than people whose metabolic rates are lower. Infants have the most slow wave sleep, and the amount declines with age (particularly stage 4 sleep). It has been suggested that this parallels the decline in cerebral and body metabolism that accompanies old age. High expenditure of energy during the day-for example, after sustained exercise in a fit person-is associated with both increased duration of sleep and increased slow wave sleep. Sleep deprivation is followed by increased amounts of slow wave sleep, perhaps as a consequence of the delayed drop in metabolic rate that normally accompanies sleep. People who sleep for a long time have high body temperatures during the day, and so their metabolic rates may be raised as well. In summary, therefore, the primary function of sleep is to preserve energy. rheories ofrestoration Types of research the results of which have built a "wall of evidence" to support the restorative theory of sleep. Cell mitoses during sleep and wakefulness. The most widely held theory about the function of sleep is that it serves as a period of recuperation or restoration. There are two ways in which this hypothesis is interpreted: total body restoration and neurological restoration. Total body restoration The first hypothesis is that sleep is a process by which the whole body (including the central nervous system) may be restored. This theory is based on an accumulation of evidence rather than on a single critical observation. When the body is in a state of catabolism the consumption of oxygen increases. It is lower during sleep than wakefulness, and lowest during slow wave sleep. Paradoxically it is during this period oflow oxygen consumption that anabolism is thought to take place. Low metabolic rates during sleep allow the net concentration of protein to increase as a result of both an increase in synthesis and a reduction in degradation. Though the processes of catabolism and anabolism are continuous, the relative rates vary according to whether the subject is awake or asleep, and it has been shown that the rate of anabolism is at its peak during sleep. Growth hormone is released mainly at night, also in association with slow wave sleep. Direct measures ofbone growth in adolescents show that sleep is associated with anabolism. Furthermore, treatment of short stature by growth hormone is more effective if the growth hormone is given at night rather than during the day. When the need for growth is great both the duration of slow wave sleep and the overall amount of sleep is increased-for example, during pregnancy, after exercise or loss of sleep, in hyperthyroidism, and during adolescence or the refeeding of patients with anorexia. Conversely, when less energy is expended-as in hypothyroidism-the amount of slow wave sleep is reduced. During periods of protein degradation ATP from cells is consumed. If protein synthesis predominates over protein degradation during sleep ATP concentrations should increase, and this is indeed the case. These observations are supported by the finding across species that cell mitosis is at a peak during sleep. There are several variations on the theme of total body restoration. It has been postulated that slow wave and REM sleep have different restorative functions, slow wave sleep being important for macromolecular synthesis and REM sleep for removing the synthetic products of slow wave sleep to maintain synaptic connections. It has also been suggested that during REM sleep neuronal connections in the catecholamine system are formed and that this activity is necessary to maintain cognitive function. BMJ VOLUME 306 6 FEBRUARY 1993384