Election Section

WASHINGTON — 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. “Both studies have to be respected.” 

But Norton said the findings only “hint” at an effect on breast cancer rates from nighttime work and “raises questions that must be addressed with more research.” 

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. For women who worked at night for more than three years, the relative risks went up to 60 percent. 

The Brigham & 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. 

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 the bright lights during the dark hours. 

Melatonin is produced by the pineal gland during the night. Studies have shown that bright lights reduces the secretion of melatonin. In women, this may lead to an increase in estrogen production and increased estrogen levels have been linked to breast cancer. 

“If you exposed someone to bright light at night, the normal rise in melatonin will diminish or disappear altogether,” said Davis. “There is evidence that this can increase the production of reproductive hormones, including estrogens.” 

Davis said changes in melatonin levels in men doing nighttime shift work may increase the risk of some types of male cancer, such as prostate, but he knows of no study that has addressed this specifically. 

Both Laden and Davis said the melatonin-estrogen-breast cancer connection is still a theory that will require more research to prove or disprove. 

Dr. S. Eva Singletary, a breast cancer specialist at M.D. Anderson Cancer Center in Houston, said the two studies show “a small relative increase in breast cancer risk, but nothing to become alarmed about.” 

More study is needed to precisely define the risk of shift work and how that compares to other known breast cancer risk factors, such as family history, smoking and obesity, said Singletary. But she said the finding does suggest the need for women who work night shifts to be particularly prudent in following breast cancer screening recommendations calling for regular mammograms and medical exams. 

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On the Net: 

American Cancer Society: www.cancer.org 

Journal of the National Cancer Institute: http://jnci.oupjournals.org/ 

National Cancer Institute: http://www.nci.nih.gov/