WASHINGTON (AP) -- Breast cancer risk increases by 8 percent to 60 percent for women who work the night shift for many years, according to two studies that suggest the bright light at night diminishes the body's supply of melatonin and increases estrogen levels.
Researchers said the fact that two independent studies, using different methods, found roughly the same results suggest strongly that working the graveyard shift for long periods of time may lower the body's resistance to breast cancer and, perhaps, to other types of 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 before a compelling case could be made to change night work schedules, however.
"The numbers in our study are small, but they are statistically significant," said Francine Laden, a researcher at Brigham & Women's Hospital in Boston and co-author of the second study.
Both studies appear Wednesday in the Journal of the National Cancer Institute.
"These studies are fascinating and provocative," said Larry Norton of the Memorial Sloan-Kettering Cancer Center in New York. But Norton said the findings only "hint" at an effect on breast cancer rates from nighttime work.
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