Objectives Animal evidence suggests that circadian disruption may be associated with ovarian cancer, though very little epidemiological work has been done to assess this potential association. We evaluated the association between self-reported nightshift work, a known circadian disruptor, and ovarian cancer in a population-based case-control study.

Methods The study included 1101 women with invasive epithelial ovarian cancer, 389 women with borderline epithelial ovarian tumours and 1832 controls and was conducted in western Washington state. Shift work data were collected as part of inperson interviews.

Results Working the nightshift was associated with an increased risk of invasive (OR=1.24, 95% CI 1.04 to 1.49) and borderline (OR=1.48, 95% CI 1.15 to 1.90) tumours; however, we observed little evidence that risks increased with increasing cumulative duration of nightshift work, and risks were not elevated in the highest duration category (>7 nightshift work-years). Increased risks were restricted to women who were 50 years of age and older and to serous and mucinous histologies of invasive and borderline tumours. There was suggestive evidence of a decreased risk of ovarian cancer among women reporting a preference for activity during evenings rather than mornings.

Conclusions We found evidence suggesting an association between shift work and ovarian cancer. This observation should be followed up in future studies incorporating detailed assessments of diurnal preference (ie, chronotype) in addition to detailed data on shift schedules.