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Long-term night work doubles breast cancer risk

Risk higher for all women on nights for 30+ years – not just nurses

Louise Prime

Tuesday, 02 July 2013

Women who work long-term night shifts have double the risk of breast cancer of their peers, research has shown. The study, published online today in Occupational and Environmental Medicine, confirms that the association already found in nurses is also seen in several other occupations.

Previous research among nurses had demonstrated associations between working night shifts and breast cancer, but there had been little work looking at other professional groups. So researchers in Canada looked at the relationship between shift work and breast cancer risk in the general population of Vancouver, British Columbia and Ontario, as well as considering tumour hormone receptor status.

They compared career histories of 1134 women with breast cancer with those of 1179 women of the same age who did not have cancer, as well as checking their hospital records for information on tumour type.

Women who had worked night shifts for up to 14 years were at no higher risk of developing breast cancer than controls; nor were women who had worked nights for 15-29 years. But after controlling for potential confounding factors, women who had worked nights for 30 years or longer had, overall, a more than doubled risk of breast cancer (odds ratio 2.21). This was seen whether they worked in healthcare (OR 3.11) or non-healthcare occupations (OR 2.25).

The risk associated with long-term night work was also higher in women whose tumours were sensitive to oestrogen and progesterone.

The study authors said that although melatonin has previously been implicated in the association between breast cancer and shift work, other relevant factors might include sleep disturbances, upset body rhythms, vitamin D or lifestyle differences.

They concluded: “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.”

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