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Researchers found that women who work at least three night shifts a week are twice as likely to develop breast cancer.
Researchers have found that women who work at least three night shifts a week for around six years or more are twice as likely to develop breast cancer.
The study, published online in Occupational and Environmental Medicine, followed more than 18,500 women working for the Danish Army from 1964 to 1999, all of whom had been born between 1929 and 1968.
It found an increased breast cancer was strongest among those who describe themselves as "morning" people or "larks," rather than "evening" people or "nightowls," according to UPI.
Lead author Dr Johnni Hansen, of the Institute of Cancer Epidemiology, in Copenhagen, Denmark, wrote that there was little to no risk for women who worked one to two night shifts per week as this workload would not change the timing of melatonin production and thereby not initiate circadian disruption.
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"The observation that women with night work and morning preference (who may be less tolerant of night shift work) tend to have a higher risk for breast cancer than similar women with evening preference warrants further exploration in larger studies," Hansen said.
Professor Jim Horne, of the Sleep Research Centre at Loughborough University, told the Telegraph, that for most women the increased risk of developing breast cancer after many years of shift-work, is still small. He added the uptake in the number of breast cancer cases in self-described larks may be due to their inability to cope with the stress of working late nights.
Dr Rachel Greig, Senior Policy Officer at Breakthrough Breast Cancer, added, “We know that shift work is associated with an increased risk of breast cancer, and this study further supports this view."
Hansen told MSN that enough women around the globe work enough night shifts to merit validating the study.
"About 10 to 20 percent of women in modern societies have night shift work. It might therefore be one of the largest occupational problems related to cancer," Hansen said.
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