Working night shifts does not increase the risk of breast cancer, according to a 10-year study of more than 100,000 women by the Institute for Cancer Research in London. 

The findings, published in the British Journal of Cancer, come as the International Agency for Research on Cancer begins to review the global evidence on night shift work and cancer — including breast cancer risk. For decades, studies have suggested that night shift work may increase a woman’s breast cancer risk, with the IARC concluding in 2007 that shift work disrupting the body’s sleep-wake cycle was “probably carcinogenic.” However, the evidence has been inconclusive.

In this comprehensive new analysis, researchers studied data from 103,000 women from the Breast Cancer Now Generations Study, following participants for 9½ years, on average, to identify who went on to develop breast cancer. They gathered information about the job histories and working patterns of the women and found no overall link between night shift work and the likelihood of developing breast cancer.

“We hope these findings will reassure the hundreds of thousands of women working night shifts that it’s unlikely their job patterns are increasing their risk of breast cancer,” said Baroness Delyth Morgan, chief executive at Breast Cancer Care and Breast Cancer Now, which funded the study.