Circadian Insomnias

Over the last two decades, circadian rhythm sleep disturbances have risen rapidly in prominence as evidenced by their full inclusion and description in the modern International Classification of Sleep Disorders (63). These disorders are classified into six subgroups (NODS is not pertinent)(Figure 1), but can be regrouped in different ways. Previously, we have grouped the disorders according to the type of chronobiotic action required to readjust or synchronize the disturbed rhythm (9). Here, the disorders are first grouped according to "natural" versus "artificial occurrence" and second according to the natural occurrence with age. Time Zone Change and Shift Work Sleep Disorder are clearly artificially induced, and will not be dealt with here. Delayed and Advanced Sleep Phase Syndromes (DSPS and ASPS) and Irregular Sleep-Wake Pattern (ISWP) occur naturally while Non-24-Hour Sleep-Wake Disorder can occur naturally or by accident as in many cases of the totally visually impaired. These represent the circadian insomnias and it is their recognition that has risen in importance in the context of the sleep disorders clinic. In the circadian insomnias the basic sleep mechanism is essentially functionally intact, but there is a temporal misalignment between the timing of the sleep-wake cycle and the norms of society for work and rest. This contrasts with the situation of the classic insomnias which are found under the headings of two forms of dyssomnia: Intrinsic and Extrinsic Sleep Disorders depending upon whether the cause is due to endogenous or exogenous causes. However, what both intrinsic and extrinsic groupings have in common is that the disturbed sleep is due to changes to the sleep mechanisms per se.

Figure 1. Diagrammatic summary of the 1990 International Classification of Sleep Disorders.
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