Symptoms of depression and insomnia in older age: A within-individual analysis over 20 years.

BACKGROUND: Depression and insomnia often co-occur, and precede one another. Possibly, insomnia gives rise to depression, and vice versa. We tested whether insomnia symptoms of an older individual are associated with later depressive symptoms in that older individual, and vice versa. METHODS: We performed a longitudinal analysis of data from a prospective cohort study in a large sample of community-dwelling older people (N = 3081), with measurements every three years, over a time period of 20 years. The within-individual longitudinal reciprocal relationship between symptoms of depression (Center for Epidemiological Studies Depression Scale), and symptoms of insomnia (three-item questionnaire, including difficulty initiating sleep, nightly awakenings, and early morning awakening) was modeled by means of a bivariate linear growth model. We tested whether symptoms of insomnia were associated with symptoms of depression three years later, and vice versa. RESULTS: Severity of symptoms of depression and insomnia and their within-individual average change over time were moderately correlated (correlation of intercepts: rho 0.41, 95% CI: 0.36 to 0.46 p < 0.001; correlation of slopes: rho 0.39, 95% CI: 0.25 to 0.52, p < 0.001). Symptoms of depression were not found to be associated with an additional risk of higher symptoms of insomnia three years later, and vice versa (p = 0.329 and p = 0.919, respectively). Similar results were found when analyses were corrected for covariates. CONCLUSIONS: In older individuals, depression and insomnia are associated and tend to increase concurrently over time, but constitute no additional risk for one another over repeated three-year intervals. These findings contradict previous research that suggests that depression and insomnia are risk factors for one another over time. The current study stands out due to the longitudinal within-individual statistical approach, but is limited by the three-year interval between measures.