Objectives: To determine (1) whether insomnia is a factor related to the presence or persistence of depression for 2 years in the Japanese population and (2) which component of insomnia is associated with the presence of depression for 2 years in a rural cohort.
Method: This is a community-based longitudinal study. Two thousand eight hundred twenty-five people aged 20 years or older were evaluated at baseline, and of those participants, 1,577 (56%) were reevaluated after 2 years. During both surveys, the participants were asked to describe demographic variables and to fill out self-rating scales of insomnia (Pittsburgh Sleep Quality Index [PSQI]) and depressive symptoms (Center for Epidemiologic Studies Depression Scale).
Results: The results of a multiple logistic regression analysis showed that depression (OR = 6.0; 95% CI, 4.4-8.0) and insomnia (OR = 2.1; 95% CI, 1.5-2.8) at baseline were significantly associated with the presence of depression at the follow-up. Most of the PSQI subscales, except for sleep duration and habitual sleep efficiency, were significantly associated (P < .01) with the presence of depression at the follow-up. In addition, the new appearance and repeated existence of depression at the follow-up were related to persistent insomnia (adjusted ORs = 7.0 and 3.3 [P < .001], respectively). A result of the receiver operating characteristic curve showed that persons with insomnia whose PSQI scores exceeded 8 points at the baseline were most likely to still have insomnia at the follow-up (cutoff point = 7.5).
Conclusions: On the basis of our results in a Japanese population, insomnia with high severity level could be a risk factor for the presence/persistence of depression in the long-term prognosis.
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