Women who work shifts rotating between days and nights for at least six years increase their risk of having a heart attack by 50 percent, according to a Harvard School of Public Health report released today.

The Nurses' Health Study, which has monitored 121,700 nurses since 1976, provided the data for the new findings, according to a press release.

The increased risk appears to be the result of greater pressure on the body caused by the inconsistent sleeping patterns of rotating shift workers, said Dr. Ichiro Kawachi, assistant professor at the School of Public Health.

"Shift work is a kind of stress," Kawachi said. "It disrupts the body's circadian rhythms. You lose sleep. The body responds by pumping out hormones like adrenaline and cortisone."

The physiological changes caused by these hormones may be the source of the increased heart attack risk, he added.

Shift work may also cause disorders such as insomnia and ulcers, according to Dr. Gary S. Richardson, director of the sleep disorders service at Brigham and Women's Hospital.

The study found that female shift workers tend to smoke and drink more and are more overweight than non-shift workers, factors which increase the risk for heart disease, Kawachi said.

But Kawachi said researchers adjusted for these contingencies in their interpretation of the data. "That's really the power of this study over others," Richardson said.

Kawachi stressed the risk of shift work, independent of other factors.

"Over and above changes in behavior, there's something else that puts people at risk as a result of doing this kind of work," he said.

The study is the first to examine the relationship between shift work and heart attacks in females, according to Kawachi. Men were the subject of a previous study.

"The report by Kawachi is the first significant study of the subject in the U.S.," Richardson said. "There have been others in Europe."

Kawachi advised caution before taking action based on his report.

"Two positive studies are not sufficient evidence to formulate public policy," he said. "We should be somewhat concerned, but we have to be cautious about what we do about it."

Kawachi said researchers will test the results in a study of 117,000 younger nurses, ranging in age from their 20s to their 40s.

"[Similar findings] would strengthen our conclusions a lot more," he said.

Nothing that health risks do not appear until a woman has worked a rotating shift for several years, Kawachi and Richardson advised similar ways to counteract its effects

"The best they could do is to do the normal things that doctors recommend to avoid heart attacks," said Kawachi

Such precautions might include exercising frequently, quitting smoking, checking their blood pressure regularly and sleeping more, he said