WASHINGTON, DC (WUSA) -- According to a new study, women who typically work the night shift more than twice a week are at a higher risk of developing breast cancer. 

The Danish study, published today in Occupational and Environmental Health, examined medical records of 18,500 women between 1964 and 1999. They found women who worked the night shift occasionally throughout their lives were not at an increased risk, but women who worked overnight hours three times a week for more than six years doubled their risk of breast cancer.

Researchers believe sleep deprivation in conjunction with constant exposure to bright light during night-time hours disrupts production of melatonin, the hormone that surpresses tumors in the body. Additionally, the late-night light exposure may lead to circadian disruption, which causes the master clock in the brain to become desynchronized with clocks in different organs, such as the breast.

The findings also revealed that night workers who considered themselves to be "morning people," and might be more stressed by working overnight hours, are four times more likely to develop the disease.

There have been many studies that suggest working overnight can increase cancer risk due to disruption of the body clock and hormones, but this new study, backed by the Danish Cancer Society, presents specific evidence that supports this view.