The research, led by Queen’s University in Kingston found that nurses, care workers, call-centre handlers, cleaners, some shop workers, and others who work night shifts for a long time can have a twice as high risk of developing the disease than those who don’t.
Experts say that disruption of the normal sleep-wake cycle affects production of melatonin – sometimes known as the ‘sleep hormone’ – that is thought to have anti- cancer qualities.
There have been some doubts about the link between shift work and breast cancer because of the difficulty of defining which working patterns create the risk. Also, several previous studies have concentrated on nurses rather than the general population.
The Canadian researchers examined 1,134 women with breast cancer and 1,179 women without the disease, but of the same age.
Women were questioned about their work and shift patterns and researchers also assessed the hospital records for the women who suffered from the disease.
The study, published in Occupational and Environmental Medicine, found that those who had worked nights for 30 or more years were twice as likely to have developed the disease, after taking account of potentially influential factors, although the numbers in this group were comparatively small.
No such relationship was found if women worked for less than 30 years doing shift work.
“An association between more than 30 years of night shift work in diverse occupations and breast cancer is supported here, consistent with other studies among nurses,” the authors said.
“As shift work is necessary for many occupations, understanding of 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.”
But experts cautioned that the increased cancer risk is yet to be confirmed.