Discussion
Physicians often ignore sleep disorders such as insomnia in clinical practice, in part because of increasing time constraints and because of poor preparation to deal with sleep disorders.37,38 The National Commission on Sleep Disorders Research (NCSDR) reviewed 10,000 medical records from 9 family practice clinics, finding only 123 records that contained a note about the patient’s sleep; not a single record suggested an effective response to the patient’s sleep complaint.1 Ignoring insomnia can have major implications for public health, however. Insomnia increases the risk of occupational injury and traffic accidents; a recent study concluded that driving while sleepy was at least as dangerous as the risk of driving under the influence of alcohol.39 Similarly, a 1991 national survey showed that respondents with chronic insomnia are more likely to feel sleepy when driving, to have impaired concentration, and to have problems in accomplishing daily tasks.10
Our results extend the results of previous investigations by demonstrating that insomnia is independently associated with a significant decrease in overall quality of life for patients with chronic illness. The magnitude of this decrease for severe insomnia is comparable with that observed for chronic conditions such as CHF and clinical depression. We found that such decrements worsened with increased levels of sleep disturbance and that these decreased values persisted even after accounting for medical comorbidity, depression, and anxiety. These findings are similar to the results of a recent population-based investigation, in which insomnia was associated with global decreases in HRQOL, even after excluding subjects who met DSM-IV criteria for depression or anxiety.12
Another possible explanation for the negative association between insomnia and HRQOL is that patients with insomnia overreport functional impairment in a systematic fashion. This phenomenon has been described in depressed patients as “negative thinking bias,”40 and it is possible that insomnia also leads to biased reporting. While it is difficult to rule this out, analyses of the health care behavior of patients with insomnia provides indirect evidence of the functional impairment of these patients, as measured by their increased use of medical and mental health services.8,9,13
Limitations
The limitations of our study deserve comment. First, the MOS included a select group of physicians and their patients who agreed to participate in a comprehensive study of medical care. That the MOS sampled only patients with 5 chronic conditions who were insured and had a continuous relationship with a provider in 3 large urban areas limits the ability to generalize. Noteworthy, however, is that the prevalence of mild or severe insomnia in the MOS sample (50%) was comparable with that reported in a recent study of managed-care enrollees (46%).9
Second, this analysis is based on cross-sectional data. We cannot rule out the possibility that decreased quality of life leads to insomnia (ie, reverse causality). The causal relation of insomnia to diminished HRQOL is supported by longitudinal data suggesting that insomnia is an important precursor of depression.5,41 Alternatively, insomnia may partially mediate the effect of chronic conditions such as CHF on HRQOL. Indeed, prior work has shown that worsening of chronic conditions tends to be associated with worsening of insomnia during follow-up, and vice versa.42
Third, sleep problems were assessed by self-report. No attempt was made to validate our findings with polysomnographic measurements. Polysomnography has limited value in the evaluation of insomnia; many self-identified insomniacs do not show objective sleep abnormalities on polysomnography43 and tend to have high night-to-night variability in the quality of their sleep.44 Indeed, some discrepancies may exist between self-reported sleep and laboratory data.45,46 Data obtained from MOS patients regarding other measures of sleep (including total sleep time, sleep latency, and adequacy of sleep) appear to verify the existence of sleep problems.42
Fourth, the definition of depression was based on DSM-III criteria, which differ somewhat from those of DSM-IV. We note, however, that the primary emphasis in revising the DSM-III criteria for these conditions was to improve discrimination between major depression and dysthymia,47 for both of which our analysis controlled. In addition, the diagnostic criteria for major depression and dysthymia incorporate sleep disturbance; thus, adjustment for depression would tend to reduce the estimated association between insomnia and HRQOL (as confirmed in Table 3).
Interpretation of the above findings should account for the nature of the study sample. Because all patients in the study sample had chronic conditions, average deviations in HRQOL scores were computed relative to the group with mild hypertension and without insomnia (reference group). Because even mild hypertension and its treatment may have an impact on several HRQOL domains,48 the associations between insomnia and HRQOL in the current study would probably have been even stronger if the reference group comprised patients with no chronic conditions.