Explaining social inequalities in health by sleep: The Japanese civil servants study

Background: Individuals of low socioeconomic status (SES) are likely to have poor sleep and poor health. This study aims to evaluate whether and how much of the socioeconomic differences in health are explained by sleep. Methods: The subjects were 3684 (2471 males and 1213 females) employees aged 20-65 working in local government in Japan. A questionnaire survey was conducted in January 2003. Analysis of covariance (ANCOVA) was performed to examine the association of employment-grade with sleep, measured by the Pittsburgh Sleep Quality Index (PSQI), and with health, measured by the Physical and Mental Component Summary Scales (PCS and MCS) of the Short Form-36 (SF-36). Results: In men, higher grade employees had better sleep and better health. The age-adjusted difference between the highest and the lowest grade employees was 1.9 points (95% confidence interval = 1.0-3.0) in the PCS and 3.4 points (1.8-4.9) in the MCS. The grade difference in health reduced to 1.5 points (0.5-2.5) in the PCS (21.1% reduction) and 2.0 points (0.6-3.4) in the MCS (41.2% reduction), when the PSQI global score was adjusted for. The grade differences in sleep quality contributed more to the health inequalities than sleep quantity. Among women, no significant grade differences were observed in the PSQI global score. The grade differences in the PCS and MCS were weaker and less consistent than those of men, and the differences hardly changed when the PSQI global score was adjusted for. Conclusion: Sleep quality may mediate the relationship between SES and physical and, in particular, mental health in men.