Working on a rotating night shift may increase women’s risk of developing type 2 diabetes, according to a study published in PLoS Medicine and reported by Health Day.

Researchers from the Harvard School of Public Health in Boston found that women who worked rotating night shifts (meaning they worked three or more nights a month plus days and evenings) were more likely to develop type 2 diabetes than women who worked regular hours, and the longer they worked the nontraditional schedule, the greater their risk.

For the study, researchers looked at data from two groups of women in the U.S. Nurses’ Health Studies I and II. The first study included more than 69,000 women between the ages of 42 and 67; the second study included nearly 108,000 women between ages 25 and 42. None of the women had diabetes, heart disease or cancer at the start of the study.

Researchers found that when compared with women who had not done rotating shift work, women who did one to two years of night work had a 5 percent increase in diabetes; women who worked shifts for three to nine years had a 20 percent increased risk; and women who worked 10 to 19 years on rotating shifts had a 40 percent increased risk. Finally, women with 20 or more years of experience on a rotating night shift had a 58 percent increase in type 2 diabetes.

“The association is quite strong and very consistent between the two cohorts,” said Frank Hu, MD, PhD, a senior author of the study and a professor of nutrition and epidemiology at Harvard.

In the past, studies have linked nontraditional work schedules to obesity and metabolic syndrome, a group of symptoms such as high blood pressure that is linked to an elevated risk of heart disease.

Although the study could not explain the link between night schedules and the increased risk, according to Hu, both biological and behavioral reasons could be playing a role. First, rotating shifts throw off the body’s natural alarm clock, which in turn, throws off the body’s ability to balance a need for energy. Working a rotating shift also affects eating and sleeping behavior and even whether or not someone smokes.

“Shift work is an important risk factor for obesity and type 2 diabetes,” Hu said. “This study increases the awareness of diabetes risk among people who work on a rotating shift, and the importance of diabetes screening, detection and prevention in this high risk group.”