Working night shifts for more than 30 years could double women's risk of developing breast cancer, a new study from Canadian researchers suggests.

Researchers with Queen's Cancer Research Institute, the British Columbia Cancer Agency and elsewhere, looked at 1,134 Canadian women who had developed breast cancer and compared them to 1,179 similar women without the disease.

They asked all the women about their work and shift patterns, and found that about a third in both group had a history of night shift work. The final numbers of women who developed breast cancer were thus relatively small.

But the researchers found that those women who had worked nights for 30 or more years were twice as likely to have developed breast cancer. 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.

Previous research has linked breast cancer with shift work done by nurses. This latest study also found an increased risk in women in other professions, such as cleaners, retail workers, and call centre handlers.

"An association between more than 30 years of night shift work in diverse occupations and breast cancer is supported here, consistent with other studies among nurses," the authors write in the journal Occupational and Environmental Medicine.

Because shift work is necessary for many occupations, the researchers say that understanding specific shift patterns increase breast cancer risk, and how night shift work might raise the risk for breast cancer is needed.

Night shift ‘probably carcinogenic’

The International Agency for Research on Cancer (IARC) -- the cancer arm of the World Health Organization which ranks cancer risks -- has declared working the night shift as "probably carcinogenic" to humans. It’s the same designation also given to UV rays and diesel exhaust fumes.

Scientists have long suspected that overnight work is dangerous because it disrupts the circadian rhythm, which is the body's biological clock. They also note that the sleep hormone melatonin, which can suppress tumor development, is normally produced at night.

The researchers of this study say it’s possible melatonin plays a role in any increased cancer risk, but they also that sleep disturbances could play a role too, as could a lack of vitamin D.

Or it could be that night work is linked to other lifestyle differences, such as less time for exercise or poor diet and obesity.

CTV medical specialist Dr. Marla Shapiro says that previous research has shown that even people who have been on night shifts for long periods of time and who get the recommended amount of sleep during the daytime, still never produce the same amount of melatonin as people who work during the day and sleep at night.

“The question I keep getting asked when people read these studies is: should we be adding melatonin? Should we be taking supplements? But there’s no data that answers that question,” Shapiro told CTV’s Canada AM Tuesday.

Shapiro agreed with the researchers that more work is needed to determine what changes can be made to help people who need to work night shifts keep their cancer risk low. In the meantime, she says, staying healthy is otherwise still important.

“If you’re a night shift worker, your lifestyle – your exercise, what you eat, your alcohol intake, your vitamin D intake – all those things you can control,” she said.