Night Shifts Cause Breast Cancer? Maybe Not

Some recent studies have suggested that working nights increases the risk of breast cancer. Now comes a new study that says it doesn’t. At least not in China.

Worldwide, breast cancer is the most common cancer among women. Denmark last year classified it as an occupational disease and began awarding compensation to night-working women who develop breast cancer and who appear to have no other risk factors.

Animal research has suggested that disturbances in the cycle of light and darkness could upset the balance of such body chemicals as melatonin and estrogen and thus increase breast-cancer risk.

But a new study from China, reported in the American Journal of Epidemiology, finds no connection between night-shift work and breast cancer.

“We basically found no association, even among women who had more than 25 years of shift work,” said Wong-Ho Chow, PhD, who worked on the study. Dr. Chow is a researcher with the National Cancer Institute in Bethesda, Maryland. Along with colleagues from U.S. and Chinese universities, she followed more than 70,000 women in Shanghai, tracking their work and cancer histories.

Over 10 years, about 1 percent of the women in the study developed breast cancer. But the risk appeared the same whether or not they worked nights and regardless of how long they had worked nights.

“There is really not much that you can criticize from reading the paper,” Kurt Straif, MD, told Reuters Health. Dr. Straif headed a team from the World Health Organization’s International Agency for Research on Cancer that concluded in 2007 that night-shift work was a probable cancer cause. That team took into account both animal research and studies of nurses and flight attendants.

Dr. Straif did say:

One has to keep in mind that this is the first study from a non-Western country.

And he pointed out that six of eight studies have found that night-shift work increases the risk of breast cancer by at least 50 percent.

So where does that leave us? Awaiting the results of more studies, of course. Dr. Straif said several were under way.

Related seminar: Pittsburgh Breast Imaging Seminar