Working night shifts for 30 or more years doubles the risk of developing breast cancer, and is not confined to nurses as previous research has indicated, a Queen’s University and British Columbia Cancer Agency study has found.

Shift work has been suggested as a risk factor for breast cancer, but there has been some doubt about the strength of the findings, largely because of issues around the assessment of exposure and the failure to capture the diversity of shift work patterns. Several previous studies have also been confined to nurses rather than the general population.

“As shift work is necessary for many occupations, 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,” says professor Kristan Aronson, co-authour of the study.

The researchers assessed whether night shifts were linked to an increased risk of breast cancer among 1134 women with breast cancer and 1179 women without the disease, but of the same age, in Vancouver, British Columbia, and Kingston, Ontario.

The women, who had done various jobs, were asked about their shift work patterns over their entire work history; hospital records were used to determine tumour type.

Around one in three women in both groups had worked night shifts. 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. But those who had worked nights for 30 or more years were twice as likely to have developed the disease.

The suggested link between breast cancer and shift work has been linked to melatonin, but sleep disturbances, upset body rhythms, vitamin D or lifestyle differences may also play their part, say the authors.

The other Queen's researchers involved in the study were Anne Grundy, Harriet Richardson and Sandip SenGupta. The study was recently published in the journal Occupational and Environmental Medicine.