Working night shifts for 30 years or more could increase breast cancer risk, a Canadian review suggests.

The World Health Organization's International Agency for Research on Cancer (IARC) considers shift work as a probable carcinogen based on limited evidence in humans and stronger associations in animal studies.

Nurse Nglia Buchan says her sleep and eating habits are affected by working night shifts. (CBC)

Previous research on breast cancer risks associated with night shifts were largely based on nurses.

Now Prof. Kristan Aronson of Queen's University in Kingston, Ont., has extended those findings to include prolonged shift work in non-health professions as well.

"Long-term night-shift work in a diverse mix of occupations is associated with increased breast cancer risk," Aronson and her co-authors concluded in the journal Occupational and Environmental Medicine.

The researchers also asked about lifetime occupational histories to capture both rotating and permanent night-shift schedules for more than 1,100 women with breast cancer and more than 1,100 others without the diagnosis who were the same ages and lived in Vancouver or Kingston. Most were in their mid to late 50s.

They also reviewed hospital records for information on cancer diagnoses.

About a third of women in both groups said they had a history of night shift work.

"Our ultimate aim is to prevent breast cancer all together," Aronson said in an interview Tuesday.

"So if this is truly an environmental risk factor for breast cancer, we can develop healthy workplace policy that would reduce the impact of shift work on women."

Sleep and hormone melatonin

The researchers considered other factors that can affect cancer risk, such as reproductive history, body mass index, smoking and alcohol consumption.

In Kingston, the ongoing research is looking at how current shift-work patterns may be related to cancer risk.

One hypothesized mechanism focuses on melatonin, a hormone that is blocked by light and has been suggested to be protective against cancer. Suppression of melatonin is a possible explanation, agreed Russel Reiter, a researcher at the University of Texas Health Science Center in San Antonio, who has also studied the topic.

"The strength of the study is in the number of individuals included," Reiter told HealthDay News.

Sleep disturbances, throwing off the body's internal clock mechanism and low vitamin D levels are other possibilities.

Nglia Buchan has worked night shifts as a nurse in Toronto for 14 years. She averages about five hours of sleep.

"I give up my sleep so that I can be with my children," Buchan said.

"Hoping that as they get a little older then I can try to regain my health again, do some more exercise, eat properly because you eat a lot to stay awake."

Provincial workplace insurance boards and employers are already interested in the findings and ways of reducing the impact of shift work, Aronson said.

In Denmark, 37 women who got breast cancer after working night shifts were compensated following a 2007 decision by the International Agency for Research on Cancer.

"It depends on people's value systems and judgments for at what point they will consider this causal," Aronson said.

The Canadian researchers plan to combine their updated data with groups in Europe to judge the carcinogen relationship of night shifts.

The research was funded by the Canadian Institutes of Health Research.