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Shift Work Could Increase Breast Cancer Risk for Women

Researchers may have identified another risk factor for breast cancer shift work. The findings of two studies show that women who work overnight shifts are at increased risk for breast cancer. Scientists are now puzzling over why that happens.

The two studies appear in the Journal of the National Cancer Institute. And if the findings are confirmed, researcher Scott Davis says, overnight work could turn out to be a major risk factor for breast cancer.

"I think the levels of risk that we've identified in our study are comparable to the levels of risk associated with a number of well-established risk factors for breast cancer, reproductive factors, for example, risk associated with alcohol consumption. Lifestyle factors that we know are associated with breast cancer risk are in about the same ballpark," he explains.

Researchers at the Fred Hutchinson Cancer Research Center in Seattle, Washington state, conducted the study. Investigators, led by Professor Davis, studied almost 1,500 women. They found a 40 to 60-percent increase in breast cancer risk among women on the nightshift compared to other women.

"We basically found that people who work what we call the graveyard shift - that is work at night - were at an increased risk of developing cancer. And that that risk increased as they worked more hours per week, and as they worked a greater number of years, a so-called dose-response," he says.

That means that women who worked overnights the longest were at the highest increased risk of breast cancer. In the study, Professor Davis says women in the highest risk group included those who worked the overnight shift continuously for more than three years.

Investigators think a hormone called melatonin plays a role. Melatonin helps induce sleep and the body produces it during periods of rest usually overnight. When melatonin levels are elevated, amounts of other hormones are suppressed. One of those is the female hormone, estrogen. Excess levels of estrogen have been tied to breast cancer. So scientists theorize that when women are not getting enough sleep, their estrogen levels are higher than they would be otherwise.

But Scott Davis of the Fred Hutchinson Center says other factors could play a role, including stress. Stress hormones have been tied to a heightened risk of cancer.

"A number of shift work occupations can be stressful," Mr. Davis says. "So, this is an important next step to try to find out what these findings actually mean. "

A second study was based on the medical and work histories of more than 78,000 nurses - women who are part of the large, ongoing Nurses Health Study out of Harvard University in Massachusetts. Investigators found that nurses who worked at least three night shifts per month for more than 30 years were 36 percent more likely to develop breast cancer.

But lead investigator Eva Schernhammer says the increased cancer risk is relative. "The overall public health impact is not going to be huge because we're talking about such a small sub-group of women here," she says.

Dr. Schernhammer is interested in finding out whether night shift work also increases the risk of other cancers. Seattle researcher Scott Davis sees no reason why men cannot be affected, since melatonin also suppresses male hormones that play a role in diseases such as prostate cancer.