"We saw a modest increased risk of heart disease associated with longer duration of rotating night shift work, which appears to wane after stopping shift work," said lead researcher Celine Vetter. She is an instructor in medicine at Brigham and Women's Hospital in Boston.
That increased risk ranged from 15 percent to 18 percent when compared to women who did not work rotating night shifts, the study found.
But the more time that elapsed after quitting such night shift work, the lower the risk for heart disease, Vetter said. And this "further supports the hypothesis that the risk of coronary heart disease associated with shift work might wane over time when women stopped working [such] shifts. This is a new finding," she said.
Rotating night shift work was defined as three or more night shifts per month, in addition to day and evening shifts.
The report was published April 26 in the Journal of the American Medical Association.
While the study can't prove that working rotating night shifts causes heart disease, the results are in line with previous findings, Vetter noted.
"It is possible that different work schedules might carry a different risk. And we have very little information on exact schedules, as well as work start and end times," she added.
"Future studies are needed that collect that level of detail so we can better understand which aspects of shift work are most critical," Vetter said. "We also do not well understand who is at highest risk."
Individual characteristics, such as chronotype -- an individual's built-in biological rhythm -- and variations in sleep patterns and quality might affect the risk, the researchers suggested.
"Although only a small number of women had an increased risk, and even though the absolute risk associated with shift work is small, and the contribution of shift work to coronary heart disease is modest, this is a modifiable factor, and changing schedules might help prevent coronary heart disease," Vetter said.
To pinpoint the effect of shift work on heart disease, Vetter and her colleagues collected data on nearly 189,000 women who took part in the Nurses' Health Study I and II. All of the women in the study reported their lifetime exposure to rotating night shift work.
The women also reported their heart health, including whether they had an angiogram that found heart-related chest pain, a heart attack, or procedures such as angioplasty, heart bypass surgery or stents.
The researchers used medical records and death certificates to confirm any self-reported heart attacks or deaths. The women also completed questionnaires about their known risk factors for heart disease every two to four years throughout the 24-year study period. Over that time, more than 10,000 women developed heart disease, the findings showed.
To isolate the effect of shift work, the investigators took into account a number of known risk factors for heart disease, such as smoking, poor diet, lack of physical activity and weight. Even after controlling for these risk factors, a modest increase in the risk of heart disease was seen with longer duration of rotating night shift work.
"Rotating night shift work has been previously shown to be associated with an increased risk of diabetes and cardiovascular [heart] disease," said Dr. Gregg Fonarow. He is a professor of cardiology at the University of California, Los Angeles, and was not involved with the new study.
"The mechanisms behind this association between night shift work and cardiovascular events, along with identification of strategies to mitigate this risk, require further study," he said.
Visit the American Heart Association for more on heart disease.
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