Breast cancer is the second-most common form of the disease in U.S. women.
The latest study to find a connection between breast cancer and working nights appears online in "Occupational and Environmental Medicine." Researchers compared more than 1,10 breast cancer patients with a like number of women who had no history of breast cancer.
About one-third of the women worked at jobs where they spent at least half their time on evening or night shifts. The breast cancer risk was more than twice as high for women with 30 years or more of night shift work.
The risk was higher still for women who worked in the health professions. What's the explanation?
Scientists suspect it involved melatonin, which regulates our daily body rhythms. Increased light exposure on night shifts may depress melatonin production and increase cancer risk.