Breast cancer risk increases by 8% to 60% for women who work the night shift for many years, according to the two independent studies that suggest the bright light at night diminishes the body’s supply of melatonin and increases estrogen levels.
In the first study, by Davis, researchers examined 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% more likely to have breast cancer than women who did not work such shifts. For women who worked at night for more than three years, the relative risks went up to 60%.
The Brigham & Women’s study, based on the schedules and medical histories of 78,000 nurses between 1988 and 1998, found only a “moderately increased risk of breast cancer after extended periods of working rotating night shifts.”
It found that nurses who worked rotating night shifts at least three times a month for one to 29 years were about 8% 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%. In addition, the lifetime risk of breast cancer for longtime shift workers could rise above 16%, from the 12.5% risk that American women face, as a whole.
Both studies appeared in the Journal of the National Cancer Institute
Both of the studies suggest that the increased breast cancer risk among shift workers may be caused by changes in the body’s natural melatonin cycle because of the bright lights during the dark hours.
Melatonin is produced by the pineal gland nocturnally. Studies have shown that bright lights reduce the secretion of melatonin, which may lead to an increase in estrogen production in women. Increased estrogen levels have been linked to breast cancer.
Researchers in both studies maintain however that the melatonin-estrogen-breast cancer connection is still a theory, and further research will need to be undertaken to make a more compelling case.
In the mean time, doctors suggest that women who work night shifts be prudent in following breast cancer screening recommendations calling for regular mammograms and medical exams.