Results
Baesd on our definition of insomnia, 16% of study patients had severe insomnia and 34% had mild insomnia. Patients with mild and severe insomnia were more likely to be female, nonwhite, poor, unemployed, and unmarried. They were also more likely to have clinical (or subthreshold) depression, congestive heart failure, and a greater number of medical comorbidities (Table 1).
We found a significant independent association between insomnia and HRQOL, even after we had statistically controlled for sociodemographic characteristics, health habits, BMI, medical comorbidities, and the presence of clinical and subthreshold depression. All covariates were forced into each HRQOL model simultaneously. Insomnia showed the strongest associations with mental health, vitality, and social subscales (based on the Wald statistic).
Relative to patients without insomnia, increased severity of insomnia was associated with a progressively greater deviation in HRQOL (Table 2). The decreases in HRQOL observed for mild and severe insomnia were pervasive across all SF-36 domains and were similar when the sample was restricted to those patients seen by a primary care clinician (family physician or general internist, n = 2463). The magnitude of the average deviation in physical function for severe insomnia was comparable with that observed for CHF; similarly, the average deviation in the mental health domain for severe insomnia was about 60% that observed for clinical depression.
Mild and severe insomnia were associated with diminished HRQOL across all SF-36 domains using logistic regression (relative to patients without insomnia); however, the magnitude of associations for subscales differed somewhat. In these analyses, the adjusted odds ratios for mild and severe insomnia were greatest for mental health (3.5 and 10.2, respectively), vitality (2.4 and 7.4, respectively), and general health perceptions (2.0 and 5.1, respectively). Other subscale results are available on request from the author.
The incremental effect of controlling for chronic medical conditions, depression, and anxiety on the decrements in HRQOL associated with insomnia is demonstrated in Table 3 for selected domains. Addition of medical variables reduces the strength of association between insomnia and physical function by 28% and 28%, vitality by 17% and 18%, and mental health by 12% and 12% (for mild and severe insomnia, respectively). Subsequent addition of depression and anxiety variables reduces the strength of association between insomnia and mental health by 33% and 38%, vitality by 18% and 23%, and physical function by 11% and 10% (for mild and severe insomnia, respectively). Even after accounting for depression and anxiety, both mild and severe insomnia account for significantly decreased HRQOL in this subset analysis.
In tests of interactions, the association between insomnia and HRQOL was similar across age, gender, race, education, and comorbidity (≥3 versus <3 comorbid conditions). Of a total of 64 interaction terms across all SF-36 subscales, only 3 were statistically significant and no consistent pattern was observed; this is what would be expected by chance alone.
TABLE 1
PATIENT CHARACTERISTICS ACROSS INSOMNIA GROUPS
| Variable | No Insomnia (N = 1583) | Mild Insomnia (N = 1145) | Severe Insomnia (N = 540) |
|---|---|---|---|
| Demographics | |||
| Age, mean (SD) | 53.8 (15) | 54.8 (16) | 53.1 (17) |
| Gender (% male)* | 44 | 35 | 27 |
| Race (% nonwhite)* | 18 | 22 | 26 |
| Income, mean adjusted 1985 household dollars* | $24,649 | $22,376 | $19,506 |
| Education, years (mean)* | 13.7 | 13.2 | 12.7 |
| Employed (%)* | 59 | 49 | 43 |
| Married (%)* | 62 | 58 | 51 |
| Clinical conditions, % | |||
| Hypertension* | 66 | 59 | 49 |
| Congestive heart failure* | 4 | 7 | 9 |
| Myocardial infarction† | 3 | 4 | 2 |
| Diabetes mellitus (type 1 or 2)† | 20 | 16 | 17 |
| Clinical depression* | 9 | 15 | 32 |
| Subthreshold depression* | 17 | 27 | 29 |
| Mean number of comorbidities* | 1.2 | 1.5 | 1.8 |
| *P≤.001. | |||
| †P≤.05. | |||
| SD denotes standard deviation. | |||
TABLE 2
AVERAGE DEVIATION IN HEALTH-RELATED QUALITY OF LIFE (HRQOL) DOMAINS ASSOCIATED WITH INSOMNIA IN THE MEDICAL OUTCOMES STUDY CROSS-SECTIONAL SAMPLE (N=3445)
| Average Deviation From Reference Group* | |||||
|---|---|---|---|---|---|
| HRQOL Domain | Average Score of Reference Group | Mild Insomnia | Severe Insomnia | Congestive Heart Failure | Clinical Depression |
| Physical function | 80.8 | -4.1 (0.8)† | -12.0 (1.1)† | -14.8 (1.7)† | -4.8 (1.3)† |
| Role, physical | 69.5 | -8.9 (1.4)† | -23.9 (1.9)† | -13.1 (2.8)† | -16.0 (2.2)† |
| Pain | 76.1 | -4.9 (0.9)† | -15.2 (1.1)† | 1.8 (1.7) | -8.9 (1.3)† |
| General health perception | 65.1 | -5.6 (0.8)† | -12.6 (1.0)† | -11.2 (1.5)† | -8.7 (1.2)† |
| Vitality | 64.0 | -7.2 (0.7)† | -16.0 (1.0)† | -7.5 (1.5)† | -13.0 (1.2)† |
| Social | 90.3 | -5.6 (0.8)† | -15.7 (1.1)† | -6.4 (1.6)† | -22.6 (1.3)† |
| Role, emotional | 80.8 | -9.3 (1.4)† | -18.7 (1.9)† | -3.5 (2.8) | -31.6 (2.2)† |
| Mental health | 80.0 | -6.6 (0.6)† | -14.6 (0.8)† | 0.5 (1.2) | -24.6 (1.0)† |
| NOTE: Average deviations in HRQOL for a chronic medical condition (congestive heart failure) and a chronic psychiatric condition (clinical depression) are provided for comparison. | |||||
| *These values correspond to the coefficients for insomnia and comparison conditions in HRQOL regression models, which are statistically controlled for demographic factors, health habits, obesity, other chronic conditions, disease severity, and study location; standard errors are in parentheses. All HRQOL values are scored on a scale of 0 to 100. The reference group (N = 1073) is defined as patients with mild hypertension (and no other tracer conditions) and no insomnia. For example, patients with severe insomnia at baseline experienced a 12.0-point decrement in physical functioning (on average) compared with the reference group | |||||
| †P ≤ .001. | |||||
TABLE 3
THE EFFECT OF ADDING SPECIFIC VARIABLE GROUPS ON THE AVERAGE DEVIATION IN HEALTH-RELATED QUALITY OF LIFE (HRQOL) DOMAINS ASSOCIATED WITH INSOMNIA AT BASELINE
| Physical Function | Vitality | Mental Health | ||||
|---|---|---|---|---|---|---|
| Model | Mild Insomnia | Severe Insomnia | Mild Insomnia | Severe Insomnia | Mild Insomnia | Severe Insomnia |
| (1) Insomnia only* | -9.6 | -18.5 | -11.5 | -23.8 | -10.2 | -23.7 |
| (2) Insomnia + sociodemographics† | -8.0 | -16.3 | -10.7 | -22.0 | -9.9 | -21.9 |
| (3) Insomnia + sociodemographics + health habits‡ | -7.5 | -15.1 | -10.3 | -21.1 | -9.7 | -21.5 |
| (4) Insomnia + sociodemographics + health habits + medical§ | -5.4 | -10.9 | -8.5 | -17.3 | -8.5 | -19.0 |
| (5) Insomnia + sociodemographics + health habits + medical + depression║ | -4.9 | 10.1 | -7.4 | -14.7 | -6.3 | -13.7 |
| (6) Model 5 + anxiety¶ | -4.8 | -9.8 | -7.0 | -13.4 | -5.7 | -11.7 |
| Adjusted R2 (model 5) | 0.37 | 0.31 | 0.49 | |||
| NOTE: Analysis sample is limited to those patients who completed a screening evaluation for anxiety disorders (n = 2197). | ||||||
| * Includes mild and severe insomnia. Average deviations are statistically significant at the P < .001 level for all models. Note that in the main analysis (Table 2), all variables, including terms for insomnia and potential confounders, were forced into each HRQOL model simultaneously. | ||||||
| †Sociodemographic variables include age, sex, race, education, income, and marital status. | ||||||
| ‡Health habits include alcohol use, smoking status, and exercise frequency. | ||||||
| §Medical variables include medical tracer conditions (hypertension, myocardial infarction, congestive heart failure, diabetes mellitus), including severity status, number of comorbid medical conditions (see text), and obesity. | ||||||
| ║Depression variables include current depressive disorder and subthreshold depression. | ||||||
| ¶Anxiety variable is defined as any anxiety disorder (generalized anxiety disorder, phobia, or panic disorder) over the previous 12 months. | ||||||