By Andrew M. Seaman

(Reuters Health) - People who occasionally work night shifts may be at a slightly increased risk of heart disease, according to a new study.

Nurses in the study who worked at least three nights per month were more likely to develop heart problems over the next 24 years than nurses who stuck to daytime shifts.

"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.

For the new study, Vetter and her colleagues used data from more than 189,000 women. About 40 percent 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 (CHD).

NHS participants were only asked once about their history of working night shifts, but NHS2 participants were surveyed about night shifts every two years.

During the follow-up period, there were 7,303 cases of CHD in the NHS study and 3,519 cases in the NHS2.

Overall, the risk went up with the number of years women spent covering night shifts, the researchers reported April 26 online 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% higher in those who worked night shifts for five to nine years, and 27% 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.

For example, women in NHS with at least 10 years of rotating night shifts had a 27% increased risk of coronary heart disease during the first half of the follow-up period, but only a 10% increased risk during the second half.

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.