Bedtime and sleep duration in relation to depressive symptoms among Japanese workers.

OBJECTIVES While bedtime may influence circadian rhythms, potentially leading to depression, epidemiological data on this issue are limited. We cross-section-ally investigated the association between bedtime and depressive symptoms in Japanese workers, taking sleep duration into consideration. METHODS The participants were 1,197 workers who participated in a health survey during a periodic checkup and had no history of psychiatric disease. Depressive symptoms were assessed using the Center for Epidemiologic Studies Depression (CES-D) scale. Usual bedtime and wake time were inquired about using a self-administered questionnaire, and sleep duration was calculated based on the difference between these two values. Multiple logistic regression was used to estimate the odds ratio of depressive symptoms for bedtime or sleep duration categories. RESULTS Short sleep duration (<6 hours) was significantly associated with an increased prevalence of depressive symptoms. Late bedtime was also significantly associated with an increased prevalence of depressive symptoms (CES-D score of £19); the multi-variable-adjusted odds ratio of depressive symptoms for a bedtime of 1:00 or later versus 23:00 to 23:59 was 1.90 (95% confidence interval, 1.16-3.12). After additional adjustment for sleep duration, however, the association was largely attenuated (odds ratio, 1.17; 95% confidence interval, 0.66-2.06). CONCLUSIONS Late bedtime was associated with increased prevalence of depressive symptoms, but this association could be largely accounted for by short sleep duration. Avoiding a late bedtime and obtaining a sufficient sleep duration may prevent depressive mood among workers.