A study by University of Oxford said working night shifts has little or no effect on breast cancer risk.
Working night shifts has little or no effect on a woman's breast cancer risk, new research has found, countering the World Health Organisation's 2007 review classifying shift work disrupting the 'body clock' as a probable cause of cancer.
The 2007 classification, by the International Agency for Research on Cancer, a cancer agency of the World Health Organisation (WHO), had limited evidence about breast cancer risk in humans so it was mainly based on a combination of animal and lab studies.
The new research by the University of Oxford in the United Kingdom examined whether night shift work increased women's breast cancer risk by following 1.4 million women in ten studies and seeing if they developed breast cancer.
It combined results from three large studies, studying 800,000 women from the Million Women Study, EPIC- Oxford and United Kingdom Biobank cohorts, with data from seven already published studies from the United States, China, Sweden and the Netherlands.
Compared with women who had never worked night shifts, those who had ever done night work - including those who had worked nights for 20 or 30 years - had no increased risk of breast cancer.
"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 10 studies that had published relevant data," said Ms Ruth Travis, lead author of the study and a scientist at Oxford.
Researchers found that the incidence of breast cancer was essentially the same whether someone did no night shift work at all or did night shift work for several decades.
The combined relative risks taking all 10 studies together were 0.99 for any night shift work, 1.01 for 20 or more years of night-shift work, and 1.00 for 30 or more years night shift work.
On average one in seven (14 percent) women in the UK have ever worked nights and one in 50 (two per cent) have worked nights for 20 or more years.
Each year in the UK around 53,300 women are diagnosed with breast cancer and around 11,500 die from the disease.
The research was published in the Journal of the National Cancer Institute.