Study finds long term night shift work associated with increased breast cancer risk

Friday, July 5, 2013. The results of a Canadian study published online on July 1, 2013 in the journal Occupational and Environmental Medicine reveal a significant association between long term night shift work and a higher risk of breast cancer. Although earlier research had uncovered a similar link among nurses, the current study demonstrates the association among members of the general population.

"While the biological mechanism linking night shift work with cancer risk remains unknown, the main hypothesised pathway involves melatonin," write Anne Grundy of Queen's University in Ontario and her coauthors in their introduction to the article. "Melatonin, a pineal hormone that is inhibited by light and considered a biomarker of chronodisruption, has also been suggested to have several cancer-protective properties. While increased light exposure during night shifts is thought to decrease production of melatonin, thereby increasing cancer risk, other mechanisms are also possible."

The researchers compared 1,134 women with breast cancer to 1,179 age-matched women who did not have the disease. Questionnaires completed by the participants provided information concerning years spent working night shifts.

Approximately one-third of women in both groups reported a history of night shift work. While no association was found for women who worked night shifts from zero to 14 years or 15 to 29 years, those who worked night shifts for at least thirty years had more than twice the risk of breast cancer than subjects who reported no night shift work. The risk of breast cancer among these very long term night shift workers was greater for those with estrogen/progesterone receptor-positive cancers than for those with estrogen/progesterone receptor negative cancers; however, the small number of women in these groups prevented the researchers from confirming a significant effect.

While a reduction in melatonin resulting from nighttime light exposure among night shift workers is one mechanism proposed for their greater risk of breast cancer risk, the authors remark that other mechanisms, including sleep disturbances, clock gene dysregulation and lifestyle differences, could also play a role. It has also been suggested that a decrease in vitamin D production due to reduced sunlight exposure experienced by night shift workers could contribute to increased disease risk.

"As shift work is necessary for many occupations, understanding which specific shift patterns increase breast cancer risk, and how night shift work influences the pathway to breast cancer, is needed for the development of healthy workplace policy," the authors conclude.