Study: Working Night Shift May Significant Increase Breast Cancer Risk

Women who work nights may have up to a 60 percent higher risk of developing breast cancer, according to two studies published in the Journal of the National Cancer Institute.

The two independent studies show an increased risk of breast cancer in women who worked night shifts or who were exposed to bright lights at night over a long period of time, suggesting that the more time women spend working nights, the greater their risk for breast cancer.

Researchers in both studies speculated that the increased risk of breast cancer in night shift workers may be caused by changes in the body's natural melatonin cycle because of exposure to bright lights during dark hours.

The pineal gland produces melatonin during the night and bright light reduces the secretion of melatonin. This may lead to an increased estrogen production in women, possibly raising the risk of breast cancer.

A study at Seattle's Fred Hutchinson Cancer Research Center and the University of Washington, looked at the work history of 763 women with breast cancer and 751 healthy women.

Women who regularly worked night shifts for less than three years were approximately 40 percent more likely to get breast cancer than women who didn't work nights.Women who worked the night shift for more than three years were 60 percent more likely to develop breast cancer, according to the researchers.

These researchers concluded, "The results of this study provide evidence that indicators of exposure to light at night may be associated with the risk of developing breast cancer."

The second study, performed at Brigham and Women's Hospital and Harvard Medical School in Boston, looked at medical and work histories of 78,562 nurses from 1988 through May 1998 and found 2,441 incidences of breast cancer.

Researchers found a moderately increased risk of breast cancer after extended periods of working night shifts with nurses working rotating night shifts at least three times a month for one to 29 years being about 8 percent more likely to develop breast cancer.

Women who worked the shifts more than 30 years saw their risk increase by 36 percent.

"Women who work on rotating night shifts with at least 3 nights per month, in addition to days and evenings in that month, appear to have a moderately increased risk of breast cancer after extended periods of working rotating night shifts," concluded the researchers.

Other Sources: Journal of the National Cancer Institute