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Graveyard shifts may raise risk of breast cancer

Women who work graveyard shifts may have an increased risk of breast cancer, a new study by the Fred Hutchinson Cancer Research Center shows.

Hutchinson Center scientists found that women who regularly work in the middle of the night could face an increased risk ranging from 60 percent to 130 percent. The risk increased with the number of hours and years worked on the shift, which includes the earliest hours of the morning.

"This study tells us it's important to keep looking at this (finding) carefully. But we're not at the point where we can make recommendations about changing behavior ... We wouldn't say quit your job," said Dr. Scott Davis, director of the study, which is reported in today's edition of the Journal of the National Cancer Institute.

Conducted with more than 1,600 women from King and Snohomish counties, the study is consistent with four previous research projects from other institutions regarding breast cancer and the graveyard shift.

The Hutchinson study focused both on the shift work and exposure to light, including sleep habits. Light exposure during normal sleeping hours disrupts the body's circadian rhythm, the biological clock that affects how the body functions.

Scientists know the brain's production of a key regulating hormone, melatonin, peaks in the dark of night when we are sleeping. Davis and his colleagues think exposure to light during those hours decreases melatonin production, which, in turn, stimulates increased production of estrogen, which has long been linked to breast cancer.

"Melatonin regulates other hormones in the body, including the production of estrogen. It may also affect other reproductive hormones," said Davis, who is also chairman of the epidemiology department at the University of Washington School of Public Health and Community Medicine.

A study by researchers at Brigham & Women's Hospital in Boston, also published today in the Journal of the National Cancer Institute, similarly found a link between late-night work and breast cancer.

The work histories of 78,000 nurses were studied over 10 years. The Brigham researchers found nurses who worked rotating night shifts at least three times a month for one to 29 years had about an 8 percent increased risk of breast cancer. Those who worked the shifts more than 30 years had a 36 percent increased risk.

The Seattle scientists, from the Public Health Sciences Division of the Hutchinson Center, interviewed about 800 women about their history of shift work and exposure to light during the 10 years prior to breast-cancer diagnosis. Each was in her job a minimum of six months. They were compared with about 800 other women of similar age who had not had the disease.

The work hours of study subjects at increased breast-cancer risk varied widely. It was difficult to define averages and patterns, Davis said.

But generally, he said, a woman had a 60 percent increased risk if she worked at least 1 percent of her total hours on the graveyard shift during the study period. That is the statistical cutoff point, he said.

Women who had worked the shift 5.7 hours a week or more had a 130 percent increased risk, or more than double the normal risk.

Years on the shift made a difference, too. For example, the researchers said, women doubled their risk for the disease if they worked at least 4½ years on shifts when they were awake at least from 1 to 2 a.m. three nights a week. That is about the time when melatonin production is at its peak.

"We found that a graveyard shift often is more than eight hours," said Davis. "Hopefully, our future research will tell us more about risks and patterns of shift work."

Overall, about 1 in 8 women in the United States will develop breast cancer during their lifetime, experts estimate. Nearly 239,000 new cases will be diagnosed this year, and about 40,000 will die of the disease, according to the American Cancer Society.

The Hutchinson researchers also found about a 40 percent increased risk of the disease among the relatively few women who regularly slept in bedrooms with light bright enough to read. But the results were not statistically significant, said Dana Mirick, a co-researcher. Other levels of light did not carry a risk. Nor did occasionally turning on the light, as when getting up to go to the bathroom.

The Hutchinson Center research is also consistent with several previous studies that involved blind women's risk of breast cancer. Those who were completely blind had significantly reduced risk of the disease. The perception of light is what regulates the production of melatonin, Davis explained.

Davis said it is possible that the stress of late-night shift work could increase breast-cancer risk. Many jobs involving shift work are stressful, including factory work, medicine, nursing, police, fire and rescue work. Stress is known to suppress the body's ability to fight off disease.

Previous studies also have shown that late-night shift work increases the risk of gastrointestinal problems, cardiovascular disease and premature births.

Davis and his colleagues next plan to look specifically at how levels of melatonin and estrogen are affected by disruptions in the body's normal sleep patterns.

He said future studies may also examine whether women with a family history of breast cancer are more vulnerable to the disruptions and why some people adapt well to late-night shift work.

"We're building on a progression of evidence," he said.

An editorial in the cancer journal called the new studies and other research on the subject "alarming ... regardless of the biologic cause."

"There is an urgent need for further exploration of the relationship between exposure to light at night, shift work, including timing during the night, and cancers that may be influenced by melatonin," said Dr. Johnni Hansen, of the Danish Cancer Society.