The idea that women who work regular night shifts face increased risk of breast cancer is wrong, according to a major new British analysis.
A decade ago international experts warned that shift work was a probably carcinogen because of disruption to the body clock.
But the latest analysis says this does not apply to humans.
The researchers say the 2007 ruling was largely based on animal and laboratory studies.
The new findings come from a study of some 1.4 million women including 800,000 taking part in three large research projects in the UK.
These findings have been combined with those from seven other studies in Europe, China and the USA for the report in the Journal of the National Cancer Institute.
The study found that neither women who worked night shifts for decades nor those who had done a little night work faced any increased risk of breast cancer.
Just 2% of British women have worked nights for 20 years or more – while just 14% have ever worked nights.
Researcher Dr Ruth Travis, of Oxford University, said: “We found that women who had worked night shifts, including long-term night shifts, were not more likely to develop breast cancer, either in the three new UK studies or when we combined results from all ten studies that had published relevant data.”
Sarah Williams, of Cancer Research UK, which backed the research, said: “This study is the largest of its kind and has found no link between breast cancer and working night shifts. Research over the past years suggesting there was a link has made big headlines, and we hope that today’s news reassures women who work night shifts."
Travis et al. Night shift work and breast cancer incidence: three prospective studies and meta-analysis of published studies. JNCI 6 October 2016; doi: 10.1093/jnci/djw169
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