Long-term shift work is associated with a higher risk of breast cancer for women, according to a new study.
Researchers from Queen's University in Canada found that women who had worked for 30 or more years on the night shift had a doubled risk for developing breast cancer compared with women who worked 29 or fewer years during nights.
However, researchers noted that the reason for this association is still less than clear. "While light at night and melatonin have been proposed as one pathway through which night shift work may influence breast cancer, and data from prospective studies has generally supported a protective effect of melatonin on breast cancer, biomarker studies of night work and melatonin are less consistent," they wrote in the Occupational & Environmental Medicine study. "A role for other potential mechanisms has been suggested, such as sleep disturbances, clock gene dysregulation or lifestyle differences, and these should be considered in future work."
The study included 1,134 women with breast cancer, as well as 1,179 women without breast cancer, who were all matched by age and lived in either Vancouver, British Columbia, or Kingston, Ontario.
Researchers interviewed them about their shift work experiences, and found that about a third of the women had worked night shifts before. They didn't find any associations between increased breast cancer risk and working 14 or fewer years of night shift or 15 to 29 years of night shift, but they did find the increased risk among women who worked 30 or more years of night shift.
Last year, a study in the same journal also showed a link between night shift work and breast cancer risk, though those researchers found that the risk was highest among those who reported being "morning people" and not "night people."