A new study has found a correlation between working nights and increased heart disease.

The study, published in the US journal, JAMA, showed that nurses who worked at least three nights per month have a 15% to 18% increased risk of developing coronary heart disease (CHD), the most common type of heart disease, Reuters reported.

“I think it’s an important message because it’s a potentially modifiable risk factor,” said lead author Celine Vetter, of Harvard Medical School and Brigham and Women’s Hospital in Boston.

There are a number of known CHD risk factors, such as smoking, poor diet, lack of physical activity, and elevated body mass index.

“However, even after controlling for these risk factors, we still saw an increased risk of CHD associated with rotating shift work,” said Vetter.

For the new study, Vetter and her colleagues used data from more than 189,000 women. About 40% were participating in the Nurses’ Health Study (NHS), which began in 1988. The others were in NHS2, which began in 1989.

The women entered the studies between the ages of 25 and 55. At the start, none of them had coronary heart disease, which is when the arteries that carry blood to heart muscle become narrowed or blocked.

These women reported on their coronary health, indicating whether they had an angiogram that confirmed CHD-related chest pain, a heart attack, or cardiovascular procedures such as angioplasty, coronary artery bypass graft surgery or stents.

During the follow-up period, there were 7,303 cases of coronary heart disease problems.

Overall, the risk went up with the number of years women spent covering night shifts, the researchers report in JAMA.

Compared to the risk for nurses in NHS2 who didn’t work night shifts, the risk of coronary heart disease was 12 % higher in nurses who worked night shifts for less than five years, 19 percent higher in those who worked night shifts for five to nine years, and 27 percent higher in nurses who worked nights for at least 10 years.

But the risk of coronary heart disease came back down as women quit working night shifts or retired, the researchers found.

The study can’t explain the association, but Vetter said it could be related to increased inflammation in the body and social disruption. She also said the findings may apply to people who work early morning shifts since they have to get up during the night.

Once researchers have more data, Vetter said, they will be able to design healthy work schedules.

“Hopefully we can design schedules that are healthier for the individual,” she said.