Night shift work that disrupts the internal body clock was classified as a probable carcinogen by the International Agency for Research on Cancer in 2007, and over two decades ago researchers first established a link between breast cancer, the most common cancer among women worldwide, and shift work. Since that time, interest has grown in quantifying that link, and there have been numerous analyses attempting to analyze the existing, and conflicting, epidemiologic data related to this association.

In a new study, published in the American Journal of Epidemiology, researchers from the Channing Division of Network Medicine at Brigham and Women’s Hospital used 24 years of data from more than 190,000 women enrolled in the Nurses’ Health Study and the Nurses’ Health Study II. In these cohorts, 9,541 incidents of breast cancer were diagnosed. Women in the Nurses’ Health Study (NHS) were, on average, 20 years older than women in the Nurses’ Health Study II (NHS2). Researchers found that, among the younger, NHS2 cohort of women, long–term rotating night shift work was associated with increased risk of breast cancer, particularly among women who performed shift work during young adulthood.

“In our previously published analyses of the rotating night shift work and breast cancer relationship, we observed a statistically significant increased risk in both NHS cohorts with approximately 10–12 years of follow–up. The analysis presented in this paper extends these findings with 24 years of follow–up and further explores the relationship by timing of shift work performance and hormone receptor status of tumors,” the authors write.

Researchers report that the risk of breast cancer after 20 or more years of rotating night shift work increased by up to 2–fold among the younger women in NHS2.

“An increase in breast cancer risk among women who work night shifts has previously been reported and this new research essentially confirms prior studies. However, in light of the age range of the NHS2 population, we suspect that the increased risk among this younger population is reflective of the timing of the exposure to shift work,which was accrued primarily during younger ages, and that younger women may be more vulnerable to this risk,” Schernhammer said.

Additionally, researchers report that women in NHS with 30 or more years of shift work had no increased risk of breast cancer when compared to women who were not shift–workers.

“Follow–up for this older group of women occurred primarily after their retirement from shift work,” Schernhammer said. “This is important to point out, as an association between increased risk of breast cancer and shift work is no longer seen among this group. We suspect that is because once shift work ends, the risk also diminishes. When we previously evaluated this in the same population but while the women were all still active in the work force, they too, experienced a higher risk of breast cancer.”

Researchers write that that these new results help to contextualize the short–term versus long–term associations, and suggest that there may be a period of increased risk of developing breast cancer among women who work night shifts that may wane with time, once they stop working nights. They emphasize that further studies should explore the role of shift work timing on breast cancer risk.