A new study has explored more comprehensively the controversial link between women working night shifts and breast cancer risk. The Canadian researchers aimed to assess whether night shifts were linked to an increased risk of breast cancer among 1,134 women with breast cancer and 1,179 women without the disease. All women were of the same age.
The women, who had worked at a variety of jobs, were asked about their shift work patterns over their entire work history. This may be important, say the authors, because risk factors vary according to hormone sensitivity, and the sleep hormone melatonin, disruption to which has been implicated in higher breast cancer risk among night shift workers, may boost estrogen production.
The study, published in Occupational and Environmental Medicine, found that those women who had worked nights for 30 or more years were twice as likely to have developed the disease
The associations were similar among those who worked in healthcare and those who did not. The risk was also higher among those whose tumors were sensitive to estrogen and progesterone.
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, say the authors.
"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," the authors conclude.
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Source: British Medical Journal