Abstract
PURPOSE: Health-related quality of life (HRQoL) impacts of insomnia and depression (as separated entities) have been well investigated in previous studies. However, little is known about the effect of comorbid insomnia and depression on HRQoL. This study aimed to assess the impacts of insomnia and depression, in combination or alone, on HRQoL in Australian adults. METHODS: Data used in this study were obtained from the large-scale longitudinal Household, Income and Labour Dynamics in Australia (HILDA) survey. Insomnia was defined using key insomnia criteria of DSM-V. Depression was based on validated cut-off points of the Mental Health Inventory-5 (MHI-5) (scores ≤ 62) in the base case analysis. HRQoL expressed as utility scores (ranging from 0 to 1) were measured using the Short-Form 6-Dimension (SF-6D) converted from the SF-36 and valued using an Australian scoring algorithm. Multi-level modelling was applied to assess the effect of insomnia and/or depression on utility scores. RESULTS: The study analysed 30,972 observations from 10,324 individuals (age [mean ± SD]: 45.7 ± 16.5, female: 54.6%). The proportion of individuals with insomnia only, depression only, and comorbid insomnia and depression was 11.3%, 11.6%, and 8.2%, respectively. The interaction effect suggested the combined impact of insomnia and depression on health-related quality of life beyond the sum of their individual effects. Marginal mean difference in utility scores for insomnia only, depression only, and the comorbidity relative to no insomnia or depression was -0.058 (SE: 0.003, Cohen's d: 0.420, small effect), -0.210 (SE: 0.003, Cohen's d: 1.530, large effect), and -0.291 (SE: 0.004, Cohen's d: 2.120, large effect), respectively. CONCLUSION: Comorbid depression and insomnia appear to have very large quality-of-life impacts. Furthermore, this is the first study that has estimated the magnitude of the impact of comorbid insomnia and depression on utility scores which can be utilised in future clinical or economic studies.