Sleep duration and the risk of endometrial cancer incidence among Japanese women: A pooled analysis of the Miyagi Cohort Study and the Ohsaki Cohort Study.

Objective: Endometrial cancer has been increasing worldwide, and is one of the most common female hormone-related cancers. The purpose of this study was to examine the association between sleep duration and risk of endometrial cancer among Japanese women.

Methods: We conducted a pooled analysis of two prospective studies conducted among residents of Miyagi Prefecture in rural northern Japan. A total of 36,537 women aged 40-79 years participated in the Miyagi Cohort Study in 1990 and in the Ohsaki Cohort Study in 1994. The participants responded to a self-administered questionnaire that including sleep duration. Sleep duration was assessed at the baseline using a self-administered questionnaire. The participants entered the mean integer number representing the hours of sleep taken per day during the previous year. We divided the participants into three groups (≤6 h, 7-8 h, or ≥9 h). Cox proportional hazards regression analysis was used to estimate the multivariate hazard ratios (HRs) and 95% confidence intervals (CIs) for endometrial cancer incidence.

Results: During 745,993 person-years of follow-up, we identified 146 incident cases of endometrial cancer. Compared with women who slept 7-8 h per day, the multivariate HR (95%CI) for endometrial cancer incidence was 1.07 (0.72-1.60) for those who slept 6 h or less, and 1.05 (0.57-1.93) for those who slept 9 h or longer (p-trend=0.57).

Conclusion: In analysis of two population -based prospective cohort studies conducted among Japanese women, we found no significant associations between sleep duration and the incidence of endometrial cancer.