Shift work, hCLOCK T3111C polymorphism, and endometriosis risk

Background: Endometriosis, a dysplastic disease affecting approximately 5%-10% of US reproductive-age women, has been linked to exposures indicating high circulating estrogen levels. One such exposure may be night shift work, which has been associated with menstrual disruption and increased risk of 2 other estrogen-influenced diseases, breast cancer and adverse coronary events. Methods: In this population-based case-control study, cases were 235 women aged 18 to 49 years who were enrolled in a large health-maintenance organization in the state of Washington, and who were first diagnosed with surgically-confirmed endometriotic disease between April 1, 1996 and March 31, 2001. Controls were 545 randomly selected women enrolled in the same program who did not have a history of endometriosis. Study participants were asked about night shift work in all paid full-time or part-time jobs they had worked from age 18 to the reference date. Genotypes for T3111C hCLOCK were determined for a subset of 218 cases and 456 controls. Results: Any night shift work was associated with a 50% increase in risk of endometriosis (odds ratio = 1.48 [95% confidence interval = 0.96-2.29]), and working more than half of shifts on a job at night was associated with a nearly doubled disease risk (1.98 [1.01-3.85]). Changing sleep patterns on days off was associated with further increases in disease risk. T3111C hCLOCK polymorphism was unrelated to endometriosis status and did not modify the effect of shift work on endometriosis. Conclusions: These findings suggest that some aspects of night shift work may influence the development of endometriosis.