Women who work three or more night shifts per month, in addition to day and evening working hours in that month, have a higher risk of developing type 2 diabetes than women who worked only days or evenings, a study finds.

Women who work these shifts also experienced weight gain associated with shift work, which also contributes to type 2 diabetes, investigators from the Harvard School of Public Health said.

The investigators analyzed data on more than 69,000 U.S. women, ages 42 to 67, in the Nurses’ Health Study I, which tracked from 1988 to 2008; also 108,000 women ages 25 to 42 in the Nurses’ Health Study II.

They found that the longer women worked rotating night shifts, the greater their risk of developing type 2 diabetes. Women who worked rotating night shifts gained more weight and were more likely to become obese during the follow-up.

“This study underscores the importance of improving diet and lifestyle for primary prevention of type 2 diabetes in this high risk group,” said senior author Frank Hu, Ph.D.

The study was published online in the journal PLoS Medicine.