Working at night causes circadian disruption and it has been classified as a probable carcinogen. An evening chronotype, or preference for late day activity, has been shown to increase risk for several adverse health effects, such as metabolic disorders and recently, breast cancer. To further explore this emerging area of interest, we examined the association between endometrial cancer (EC) risk, another common cancer in women, and chronotype. The women in this study were members of the California Teachers Study cohort, which was established in 1995. Chronotype was reported on a subsequent questionnaire (Q5), administered in 2012-2013. The women included in this analysis were under age 90 years, were post-menopausal at Q5, and had no hysterectomy. The cancer cases, identified through linkages to the California Cancer Registry, were diagnosed between 1996 and 2014. We used unconditional logistic regression models to estimate the odds ratios (ORs) and 95% confidence intervals (CIs) of the associations between chronotype and EC diagnosis. There were 437 EC cases and 26,753 cancer-free controls included in this analysis. Controls were more likely to classify themselves as current morning chronotypes than were cases (39% and 34%, respectively). Compared to morning types, women who were definite evening types had a statistically significantly elevated OR of 1.44 (95% CI 1.09-1.91). This association was more pronounced among obese women as compared to non-obese women. For evening type compared to morning type, the OR among obese women was 2.01 (95% CI 1.23, 3.29) while the OR for non-obese women was 1.12 (95% CI 0.77, 1.63). To our knowledge, the association between EC risk and evening chronotype has not been previously reported, but is consistent with the small body of literature which suggests increased breast cancer risks among evening chronotypes. Because this study was based on a retrospective analysis in a cohort of mostly white female teachers in California, further analysis of chronotype as a potential EC risk factor should be considered in other cohorts and in prospective analyses in order to further explore this relationship.
