Higher depressive symptoms have been reported in rotating shift workers compared with day workers. Depressive symptoms in adults who do not engage in night work have also been shown to be associated with chronotype and sleep duration. This study examines associations between depressive symptoms, morningness-eveningness (i.e. the degree to which people prefer to be active in the morning or the evening), sleep duration and rotating shift work. Japanese nurses (1252 day workers and 1780 rotating shift workers, aged 20-59) were studied using a self-administered questionnaire. The questionnaire covered depressive symptoms, morningness-eveningness, sleep habits and demographic characteristics of the participants. The Center for Epidemiologic Studies Depression Scale (CES-D) was used to determine the levels of depressive symptoms. A Japanese version of the Morningness-Eveningness Questionnaire (MEQ) was used to measure morningness-eveningness. The CES-D score of shift workers was significantly (p < 0.05) higher than that of day workers. The MEQ score was significantly (p < 0.05) lower (i.e. greater eveningness) in shift workers than in day workers. Sleep duration on the day shift was significantly (p < 0.05) shorter in shift workers than in day workers. Simple linear regression revealed that the MEQ score, sleep duration on the day shift and current work shift (i.e. rotating shift work) were significantly (p < 0.05) associated with the CES-D score. Multivariate linear regression indicated that greater eveningness and shorter sleep duration were independently associated with higher CES-D scores, while rotating shift work was not. These associations between the MEQ score, the sleep duration and the CES-D score were also confirmed in both day workers and shift workers when the groups were analyzed separately. These results suggest that greater eveningness and shorter sleep duration on the day shift were independently associated with higher levels of depressive symptoms, which may explain associations between rotating shift workers and depressive symptoms. These findings have important implications for the development of novel strategies for preventing poor mental health in day workers and rotating shift workers.
