Working night shifts for 30 or more years doubles the risk of developing breast cancer, according to Canadian researchers.
Shift work has previously been suggested as a risk factor for breast cancer, especially in several studies involving nurses.
But the authors of the new study said there had been some doubt about the strength of earlier findings, largely because of lack of data on exposure to shifts and diversity of shift work patterns.
The new study involved 1,134 women with breast cancer and 1,179 women without, who were of the same age. The women were asked about their shift patterns over their entire work history.
There was no evidence those who had worked nights for up to 29 years had any increased risk of developing breast cancer. But those who had worked nights for 30 or more years were twice as likely as controls to have developed the disease.
The suggested link between breast cancer and shift work has been put down to melatonin, but sleep disturbances, upset body rhythms, vitamin D or lifestyle differences may also play their part, said the authors online in the journal Occupational and Environmental Medicine.
“Understanding which specific shift patterns increase breast cancer risk, and how night shift work influences the pathway to breast cancer, is needed for the development of healthy workplace policy,” they said.
But Dr Hannah Bridges, senior public health and information officer at Breakthrough Breast Cancer, said it still remained unclear why night work might increase breast cancer risk.
“Shift work may lead to unhealthy lifestyle habits that could independently increase the risk of breast cancer,” she said.
In 2001, the Nurses’ Health Study reported nurses with at least 30 years of night shift work had a 36% risk of developing breast cancer, compared to 8% of those who had done less than 30 years.
Participants from that study, published in the Journal of the National Cancer Institute, were followed over a 10 year period.