Women who regularly work the night shift or work into the early hours are at a significant higher risk of developing breast cancer, warn a new Danish study.

“The results indicate frequent night shift work increases the risk for breast cancer and suggest a higher risk with longer duration of night shifts,” said Dr. Johnni Hansen of the Danish Cancer Society. “Those with morning preference tended to have a higher risk than those with evening preference.”

The new study published in the Occupational and Environmental Medicine, looked at data of more than 18,500 women who worked for the Danish Army between 1964 and 1999.

The findings demonstrated that night shift workers had on average a 40 percent higher risk of breast cancer compared with peers who worked days.

Women who worked nights three or more times a week for over six years were more than twice as likely to develop breast cancer as day worker counterparts.

Moreover, the risk was highest among women who were naturally early risers. Reportedly, women who regularly work into the early hours were nearly four times as likely to have the disease.

“Night shift work involves exposure to light at night, which decreases the production of the night hormone melatonin that seems to protect against certain cancers,” said Hansen.

In addition, light at night disrupts body’s natural circadian rhythm and desynchronized master clock in the brain from local cellular clocks in different body organs.

“Repeated phase shifting may lead to defects in the regulation of the circadian cell cycle, thus favoring uncontrolled growth,” of cells, noted Hansen.

Furthermore, sleep deprivation after night shift work negatively affects the function of immune system, which is responsible for detecting and destroying any cell with abnormal or cancerous growth.