A study published online in Occupational and Environmental Medicine shows women who work night shifts for 30 or more years have double the risk of developing breast cancer. The risk was originally thought to be restricted to those in the nursing field, but researchers have discovered the risk includes women working night shifts in various occupations.

Researchers assessed 1,134 women with breast cancer and 1,179 women without the disease, but of the same age, in Vancouver, British Columbia, and Kingston, Ontario. The women, involved in several different jobs, were asked about their shift work patterns over their entire work history.

Approximately one in three women in both groups had worked night shifts. There was no evidence that those who had worked nights for up to 14 years or between 15 and 29 years had any increased risk of developing breast cancer.

Those who had worked nights for 30 years or more, however, were twice as likely to have developed the disease.

The number of health care workers who developed cancer was comparable to those who had not worked in health care and developed the disease.