In 2007, a study has determined that working night shifts increases a person’s risk of developing cancer, including breast cancer. However, results of studies recently published in the Journal of the National Cancer Institute show that night shift work does not have any effect on breast cancer risk.

Previously, the World Health Organization’s International Agency for Research on Cancer explained that people’s internal body clocks get disrupted by night shift work, causing an increased risk of developing cancer. This report was based on an experiment conducted on animals without breast cancer.

The researchers studied 800,000 women in the UK. These participants had to answer questions regarding night shift work as well as their cancer status.

Overall, the research team found that night shift work did not actually increase the incidence of breast cancer among these participants. Even those who worked long-term night shifts do not have a higher risk of getting breast cancer.

Other studies conducted in the US, China, Sweden and in the Netherlands have also studied 1.4 million women in total, all of whom worked night shifts. In total, only 4,660 of the participants were diagnosed with breast cancer.

Overall, the risk for any night shift work is only 0.99. Meanwhile, those who worked night shifts for two decades or more only have 1.01 relative risk of getting breast cancer and those who worked for three decades and more only have 1.00 relative risk of getting cancer.  Hence, the research team concluded that working night shifts has little to no effect on a person’s risk of getting breast cancer.

“We found that women who 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,” says study researcher Ruth Travis.