At the time of the 2007 classification there was limited evidence about breast cancer risk in humans so the classification was mainly based on a combination of animal and lab studies.
The new research, published in the Journal of the National Cancer Institute and funded by the UK Health and Safety Executive, Cancer Research UK and the UK Medical Research Council, 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 new results from three large studies, studying 800,000 women from the Million Women Study, EPIC-Oxford and UK Biobank cohorts, with data from seven already published studies from the USA, 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.
Dr Ruth Travis, lead author and a Cancer Research UK-funded scientist at the Cancer Epidemiology Unit at the University of Oxford, 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 10 studies that had published relevant data.”
The 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 per cent) 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.
Professor Andrew Curran, chief scientific adviser for the the HSE, which commissioned the study, said: “Breast cancer is the most common cancer in women so it was vital for us to fund work in this area to establish if there is a link to night work.
“In Great Britain, there are 2 million women, about one in six female workers, who are currently working in some types of shift work, and over half million of them are working in shifts that involves night work.
“This study has shown that night shift work, including long-term shift work, has little or no effect on breast cancer incidence in women. However, there are a number of other known risks with shift work that employers must take into consideration when protecting their workers’ health and safety.”
Professor Cathie Sudlow, chief scientist at the UK Biobank, said: “The work demonstrates the power of very large, well-designed studies to answer important questions where previous evidence has been unclear. This study is also a landmark for UK Biobank, since it is the first time that associations between potential risk factors (in this case working nights) and new cases of cancer developing during follow-up, have been published using UK Biobank data. We expect many further findings to emerge from UK Biobank alone as well as from combining its data with other major studies, such as the Million Women Study and EPIC. Collaborative efforts like this will advance our understanding of a wide range of diseases more quickly and help find new ways to prevent and treat them.”
Sarah Williams, Cancer Research UK’s health information manager, 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.
“Breast cancer is the most common cancer in the UK and research to fully understand the different risk factors is vital so that we can give women clear health advice. Women can reduce their risk of breast cancer by keeping a healthy weight, drinking less alcohol and being active.”