Researchers from Seattle’s Fred Hutchinson Cancer Research Center compared more than 1,100 women with the most common kind of advanced-stage ovarian cancer, 390 women with early-stage cancer and 1,800 women without cancer for the study.
They say that working a night shift raises women’s risk of contracting advanced ovarian cancer by 24% and early-stage cancer by 49%.
The increased risks only applied to women aged 50 and older in the study.
Women who identified themselves as “early birds” tended to be more affected by working a night shift, while women who identified as “night owls” didn’t have significantly higher cancer risks.
The researchers suggest that the increased cancer risk could be associated with the hormone melatonin, which regulates reproductive hormones including estrogen. Melatonin is suppressed by light, and is more likely to be produced at night.
Researchers are also warning that their study only proves an association between the night shift and ovarian cancer, not a cause-and-effect relationship.
The study appears in the journal Occupational and Environmental Medicine.