Sleep and Morbidity

In CPSI and CPSII, the minimal mortality was observed 1 hr below the sample mode of 8 hr (approximately 1/2 hr below the median) (2,3). Increased mortality 

A creative series of studies found that young adult volunteers could reduce their habitual sleep from 8 hr to about 5 hr with almost undetectable effects on performance or mood, except for an increase in sleepiness (42,43). Such investigations should be repeated and expanded, to see if such sleep restriction is so free of morbid risk. 

In summary, habitual sleep durations of 5-7 hr confer little mortality risk, and perhaps no substantial morbidity, although those with short sleep durations may fall asleep more rapidly and sleep more soundly. 

In CPSII, in the 32-covariate models, reported insomnia was associated with risk ratios slightly but significantly less than 1.0, after controlling for sleep duration (3). A similar result was found in another study (52). This might imply a protective effect of insomnia. Similarly, insomnia did not predict total mortality when depression and other comorbidities were controlled in major Swedish studies (13,53). In general, studies that control well for comorbid factors do not find that insomnia predicts increased mortality independent of sleep duration and hypnotic drug use. 

The question of insomnia effects on morbidity is more difficult. Since insomnia complaints are associated with depression, anxiety, neuroticism, and a wide variety of medical illnesses (44,49,54,55), it may be difficult to distinguish effects of insomnia from effects of the comorbid processes. In some cases, medications taken by insomniacs may be responsible for impairment. One attempt to assess disability related to insomnia found no association meeting Bonferroni criteria, after adjustment for age, gender, chronic disease, and major depression (56). It is possible that the trend for association would have been entirely eliminated had control been done for subthreshold depression, which was prevalent in the sample. Although sleep symptoms do predict future depression, they are less 

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