Breast cancer risk greater for women on night shifts

Carl T. Hall SF Chronicle 17oct01

Women who are exposed to a large amount of light at night -- whether they work the graveyard shift or just endure frequent sleep interruptions -- appear to have a higher risk of breast cancer, scientists are reporting today.

In two separate studies, researchers at the Fred Hutchinson Cancer Research Center in Seattle and at Harvard Medical Center found a strong link between nighttime light exposure and elevated cancer risk.

The risk can be as much as 60 percent higher for those exposed to light at night, according to the Seattle study.

A family history of breast cancer poses a much higher relative risk, but the night-light factor appears to bring roughly the same extra risk as some other well-known breast cancer risks, including alcohol use, delayed childbirth and use of high-dose birth control pills.

The findings add an ominous new dimension to the widespread suspicion that shift work can be toxic on a number of counts by disrupting the brain's built- in day-night clock and throwing hormone levels out of whack.

Natural production of the hormone melatonin rises sharply in the evening and peaks around midnight to 2 a.m., driven mainly by the amount of light reaching the eyes.

Abnormally low levels of melatonin can lead to elevated levels of the female sex hormone estrogen. And high circulating estrogen levels are a known cancer risk.

SUNLIGHT MAY BE A FACTOR
Scientists said that unnatural light exposure at night -- combined, perhaps,

with minimal exposure to bright sunlight during the day in indoor-oriented urban societies -- may go a long way toward explaining the epidemic of breast cancer in the United States and other economically advanced countries.

"Melatonin acts as a regulator for a number of endocrine functions in the body," said Scott Davis, co-writer of the Seattle study and chairman of the department of epidemiology at the University of Washington.

"Without trying to scare people, the whole point here is that we may have identified an aspect of the environment that has an important impact on the risk of breast cancer, and possibly other diseases related to hormones. We need to follow this up and see if we can figure out exactly what it is about being up at night that causes an increased risk."

SUPPLEMENTS NOT RECOMMENDED
Researchers said there's still no evidence that the use of melatonin supplements are worth the risk. Nor did they call for any other immediate lifestyle changes, other than to suggest that everyone turn out the lights and pull the blinds when they sleep.

"It's a good idea to get what I call a dark night's sleep as often as you can," said Richard Stevens, an epidemiologist at the University of Connecticut and co-author of the Seattle study.

The findings were based on interviews with 814 breast cancer patients asked to report how often they had worked the graveyard shift -- starting work after 7 p.m. and stopping before 9 a.m. -- during the 10 years before their illnesses were diagnosed.

The more night shifts worked per week, and the more years the women had spent working at night, the higher the breast cancer risk.

SLEEPLESS WOMEN AFFECTED
Women who reported not sleeping through one or more nights per week also had a higher cancer risk -- 14 percent for each sleepless night. There was even a hint, although not statistically significant, that women with more brightly lit bedrooms ran a higher risk of cancer.

Similar conclusions were drawn by the Harvard researchers from data gathered as part of the famed Nurses' Health Study, a continuing effort to track the health status of nearly 80,000 female nurses.

The nurses were first interviewed in 1988 and were followed for the next 10 years, during which 2,411 were found to have breast cancer.

Those who had worked at least three rotating night shifts for the longest time period -- 30 years or more -- had a 36 percent increased risk of breast cancer compared with those who worked no nights at all.

Both studies appear today in the latest issue of the Journal of the National Cancer Institute, a part of the National Institutes of Health, which helped finance the work.

EARLIER RESEARCH CONCURS
Earlier studies of cancer risks in flight attendants and telecommunications workers, who also work frequently at night, produced evidence also tending to support the "melatonin hypothesis." And a 1998 study of breast cancer in blind women, whose melatonin output is not affected by light hitting receptors in the eye, found they had a 20 to 50 percent reduced risk.

"The theory behind it is pretty solid," said Dr. Debu Tripathy, associate professor of medicine and breast cancer specialist at the University of California at San Francisco.

He added, however, that it's too soon to draw any firm conclusions as to the magnitude of the effect of light exposure. Nor would it be appropriate to recommend any lifestyle changes for shift workers.