Bonnet(9) has comprehensively reviewed the clinical and experimental evidence that sustained sleep disturbance can have serious adverse psychological effects. The term sleep disturbance covers the following.
• Impaired quality of sleep (disruption of sleep architecture at either the macro- or microstructural level of sleep, or both).
• Inappropriate timing of the sleep period in relation to sleep-wake rhythms (as in the various circadian sleep rhythm disorders such as jet lag, shift work, or the more persistent forms seen in clinical practice).
Experimental studies of total sleep loss demonstrate a progressive deterioration in cognitive function, mood, and behaviour related to length of sleep loss. However, inter- and also intraindividual differences in susceptibility are seen reflecting such factors as motivation, personality, and usual sleep requirements. Task characteristics, timing of the task in relation to the circadian sleep-wake rhythm, and physical environmental factors, such as noise and other distracting stimuli, are also important.
Variations for similar reasons are reported in partial sleep deprivation experiments which (like those concerning fragmentation of sleep) are much closer to real-life sleep disturbance caused by social activities, job demands and other aspects of modern lifestyle. These studies raise the issues of how much sleep is needed for optimal daytime functioning and whether these requirements are not being met. It has been argued that there is a 'national sleep debt' in the United States (and other Western countries) and that by sleeping longer than they do habitually, many people would increase their performance and improve their well being during the day. (13) This view has been contested.(14)
Whatever the size of the sleep deprivation problem in the community, experimental evidence does suggest that a consistent reduction of the total sleep time by more than about 1 to 2h compared with the individual's unrestricted sleep period is likely to affect daytime performance and behaviour. This is in keeping with the suggestion that the first few overnight NREM-REM sleep cycles (core sleep) are the most important part of sleep for brain restitution rather than either NREM or REM sleep individually.(15) As already mentioned, however, continuity of sleep is important, as well as timing of the sleep phase.
The usual subjective effects of sleep disturbance are mood changes, especially irritability, and complaints of fatigue and poor concentration. More dramatic effects are described with prolonged and severe sleep disturbance such as disorientation, illusions, hallucinations, persecutory ideas, and inappropriate behaviour with impaired awareness (automatic behaviour) caused by frequent microsleeps. Psychometric studies have shown that sleep disturbance can produce a range of cognitive impairments, again depending on its duration and individual susceptibility. Sustained attention (vigilance) is particularly vulnerable and possibly divergent intelligence or creativity/16 A recent meta-analysis of research findings in adults indicated that the mean level of functioning of sleep-deprived subjects was equivalent to only the ninth percentile of non-sleep deprived subjects. In addition, however, mood was more affected by sleep deprivation than cognitive performance and much more than psychomotor function, which, however, was still significantly worse than in the non-sleep-deprived groups. (1,Z)
The experimental findings are in keeping with the results from studies of various occupational groups, including junior hospital doctors (1,8) and drivers of various types of vehicle/19 in which reduced performance or accidents are associated with sleep disturbance. The common and increasing practice of shiftwork is contrary to the fundamental biorhythm of sleeping at night and being awake during the day, and is often accompanied by a reduction in total sleep time and poor-quality sleep. It is not surprising that working shifts commonly results in loss of well being, physical complaints, and impaired productivity and safety. Similarly, the distribution over the 24-h period of road accidents (especially those not involving other vehicles) and other mishaps at work, corresponds to that of the levels of sleep tendency assessed objectively. Even industrial and engineering disasters have been attributed to sleep loss and impaired performance on the part of key personnel. (29
Additional evidence that sleep disturbance affects daytime function comes from neuropsychological studies of certain sleep disorders. The impaired performance on prolonged and complex tasks of subjects with narcolepsy has been shown to be secondary to the effects of their daytime sleepiness rather than to an intrinsic neurological deficit. In adult patients with obstructive sleep apnoea, attention and memory impairments (like the depression and irritability commonly reported by these patients) are also largely attributable to daytime sleepiness and the limitations that this imposes. However, there is some evidence that their deficits in more complicated 'executive functions' (formulating goals, planning and carrying out plans effectively) are not necessarily reversed when their sleepiness is relieved by treatment. This might be the result of irreversible anoxic brain damage in the later stages of the condition. (21) Clearly, early detection and treatment of this condition is essential to prevent this happening.
Both clinical and polysomnographic recovery from short periods of sleep disturbance occurs after much less sleep than that originally lost, for example, after one night's sleep following sleep loss over several days and nights. Reversal of the effects of prolonged sleep disturbance in real life is likely to be complicated, for example, by the emotional consequences of the disturbance.
Almost all the above observations about the psychological effects of sleep disturbance have been made on adult subjects or patients. The area is largely unexplored in other age groups but there is no reason why the general principles should not apply in children to) and the elderly. In one recent study of children with nocturnal asthma, it was possible to demonstrate that an improvement in the physiological quality of sleep (achieved by better asthma control) was associated with improvements in some aspects of daytime psychological function/23
Another group on whom further research is particularly required are people with mental retardation. Neglect of this group, despite the strong evidence of their very high rates of sleep disturbance,(24) is reflected in their omission from the section concerned with mental and neurological disorders in the International Classification of Sleep Disorders (described below). The available literature provides good reason to believe that the sleep disorders, especially in the more severely retarded groups, not only affect the majority but also are unusually severe and long lasting because of lack of appropriate advice and treatment. The sleep disturbance is a problem in its own right, and is often associated with various cognitive and behavioural abnormalities which might, at least partly, be the consequence of the sleep disturbance.
Improvement in the duration or the quality of sleep may be one of the few ways of improving the psychological well being of people with mental retardation (and that of their carers) whose basic condition itself cannot be improved. Contrary to the common supposition by both professionals and relatives, success can usually be achieved (even in severe and long-standing problems(25)) given an accurate diagnosis of the type of sleep disorder, which may be predominantly behavioural or physical in type depending on the cause of the mental retardation.
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