How Does Night Shift Work Influence Breast Cancer Risk?

Study challenges previous findings that suggest night shift work increases the risk of breast cancer.

A new study suggests that working the night shift has little to no effect on the risk of developing breast cancer.

This comes after another recent study showed that disrupting the circadian rhythm caused faster and more aggressive tumor growth in mouse models of non-small cell lung cancer. Circadian rhythm disruption has also been associated with sleep disorders, obesity, diabetes, depression, and bipolar disorder.

In 2007, an agency of the World Health Organization said that working night shifts and associated disruptions in circadian rhythm was likely carcinogenic. The current study, published by the Journal of the National Cancer Institute, reviewed data from 800,000 women gathered from 3 large studies conducted in the UK.

Scientists who conducted the study did not discover any increases in breast cancer risk associated with night shift work among the 3 studies. Also included in the meta-analysis were findings from 7 previously published studies combined with the findings from the 3 large studies. In total, 1.4 million women were included.

Approximately 4600 individuals who developed breast cancer had never worked night shifts, according to the study.

“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 3 new UK studies or when we combined results from all 10 studies that had published relevant data,” said researcher Ruth Travis, BA, MSc, DPhil.

Compared with women who never worked the night shift, the relative risk of developing breast cancer was 0.99 for any amount of night shift work, 1.01 for 20 or more years of the work, and 1 for 30 or more years, according to the study.

These findings suggest that the risk for developing breast cancer was nearly the same for all individuals in the study, and night shift work may not increase breast cancer risk.

“In Great Britain, there are 2 million women — about 1 in 6 female workers – who are currently working in some type of shift work, and over half a million of them are working in shifts that involves night work,” said Andrew Curran, chief scientific adviser, UK Health and Safety Executive, which funded the study. “This study has shown that night shift work, including long-term night 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 employees must take into consideration when protecting their workers' health and safety.”