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London - Women who work night shifts for more than 30 years are twice as likely to get breast cancer, scientists say.
The researchers believe artificial lighting at night interferes with chemicals in the body, and this process can trigger the growth of tumours.
The Canadian team studied the careers of 1 134 women with breast cancer and compared them with 1 179 who had never been diagnosed with the illness.
About a third of the women had done night shifts at some point during their lives working in roles such as nursing, flight crew, hotel receptionists and in hotels.
The academics then divided all the women into three groups depending on how many years they had worked nights – up to 14, 15 to 29 and 30 or more.
The findings, published on the British Medical Journal’s website, showed that women who worked night shifts for less than 30 years were no more at risk of breast cancer than anyone else.
But, if they did them for more than 30 years, they were twice as likely to develop the disease.
Although many people work under strip lighting at all hours of the day, the scientists think it is only artificial light at night that triggers cancer growth because it upsets the body’s natural sleep cycles and hormone rhythms.
They think artificial lighting suppresses the production of melatonin, a hormone that can prevent cancer. This then leads to over-production of the hormone oestrogen, which is known to trigger cancer.
A number of previous studies have linked shift work to breast cancer. But many were carried out on nurses, leaving researchers unsure as to whether only hospital workers were susceptible or if women in other jobs were also at risk.
But, this study suggests that the risk of developing breast cancer has more to do with the number of years on night shifts rather than the type of job itself.
Co-author Dr Kristan Aronson, of Queen’s University, Kingston, Ontario, said further work was needed to understand exactly what in night shifts was triggering cancer.
Once this was known, offices, hospitals and other workplaces could be adapted to help reduce the risk to women.
The report states: “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.’
In 2009, 37 Danish women were compensated by their government after developing breast cancer on night shifts.
Denmark is the only country in the world where breast cancer resulting from shift work is recognised as an industrial injury.
Despite the evidence, many academics remain unconvinced that night shifts cause breast cancer.
Dr Jane Green, an expert in the study of diseases at the University of Oxford, said: “The finding of an increased risk of breast cancer in women with a long history of shift work adds to similar results from some previous studies, but does not change the existing consensus.
“While there is some evidence to associate increased risk of breast cancer with very long-term shift work, the evidence is not yet sufficient to be sure, and certainly not sufficient to give a public health message about working shifts.” – Daily Mail