Washington Women who work nights may increase their breast cancer risk by up to 60 percent, according to two studies that suggest bright light in the dark hours decreases melatonin secretion and increases estrogen levels.
Two independent studies, using different methods, found increased risk of breast cancer among women who worked night shifts for many years. The studies, both appearing in the Journal of the National Cancer Institute, suggested a "dose effect," meaning that the more time spent working nights, the greater the risk of breast cancer.
"We are just beginning to see evidence emerge on the health effects of shift work," said Scott Davis, an epidemiologist at the Fred Hutchinson Cancer Research Center in Seattle and first author of one of the studies. He said more research was needed, however, before a compelling case could be made to change night work schedules.
"The numbers in our study are small, but they are statistically significant," said Francine Laden, a researcher at Brigham and Women's Hospital in Boston and co-author of the second study.
In Davis' study, researchers explored the work history of 763 women with breast cancer and 741 women without the disease.
They found that women who regularly worked night shifts for three years or less were about 40 percent more likely to have breast cancer than women who did not work such shifts. Women who worked at night for more than three years were 60 percent more likely.
The Brigham and Women's study, by Laden and her colleagues, found only a "moderately increased risk of breast cancer after extended periods of working rotating night shifts."
The study was based on the medical and work histories of more than 78,000 nurses from 1988 through May 1998. It found that nurses who worked rotating night shifts at least three times a month for one to 29 years were about 8 percent more likely to develop breast cancer. For those who worked the shifts for more than 30 years, the relative risk of breast cancer went up by 36 percent.
The studies relate to working hours between 7 p.m. and 9 a.m. on shifts that include the peak melatonin secretion time of about 1:30 a.m., the researchers said.
American women have a 12.5 percent lifetime risk of developing breast cancer, according to the American Cancer Society. Laden said her study means that the lifetime risk of breast cancer for longtime shift workers could rise above 16 percent. There are about 175,000 new cases of breast cancer diagnosed annually in the United States and about 43,700 deaths. Breast cancer is the second only to lung cancer in causing cancer deaths among women.
Both of the Journal studies suggested that the increased breast cancer risk among shift workers is caused by changes in the body's natural melatonin cycle because of exposure to bright lights during the dark hours.
Melatonin is produced by the pineal gland during the night. Studies have shown that bright light reduces the secretion of melatonin. In women, this may lead to an increase in estrogen production; increased estrogen levels have been linked to breast cancer.
Both Laden and Davis said the melatonin-estrogen-breast cancer connection is still a theory that will require more research to prove or disprove.