Women who work during the night shift are not likely to increase their risk of breast cancer, contrary to what researchers initially thought.
Breast Cancer Now, a UK-based charity organization, funded new research to determine how shift work can affect the health of women. Researchers found working during the evening does not increase the likelihood of women to develop breast cancer.
The findings are contradictory to those presented in a 2007 International Agency for Research on Cancer report, which suggested that disrupting the body's sleep-wake cycle because of shift work was probably carcinogenic.
"A possible link between exposure to electric light at night and an increased risk of breast cancer was first proposed more than 30 years ago, but research has so far been inconclusive," said Michael Jones, a staff scientist at the Institute of Cancer Research and co-author of the Breast Cancer Now study.
"In our new study, we found no overall link between women having done night shift work in the last 10 years and their risk of breast cancer, regardless of the different types of work they did involving night shifts, and the age at which they started such work."
Breast Cancer Risk Among Women
The 2007 IARC report described a potential link between shift work and an increased risk for breast cancer among women. Its findings were later used as the basis for the Danish government to provide payouts to dozens of women, who claimed that they developed the disease because of working during the night shift.
In the latest study, the Breast Cancer Now researchers built on the conclusions outlined in 2016 meta-analysis, which found little to no association between shift work and the disease. This earlier research was challenged because of the older average age of participants and the limited details it provided regarding the nature of shift work for women.
To address these issues, Jones and his colleagues recruited participants with an average age of 45 years. As much as 17.5 percent of these people reported that they were night shift employees, whose regular working hours were between 10 p.m. and 7 a.m. over the last 10 years. The participants' shift data were also followed up six years later.
No Association Between Shift Work And Breast Cancer Risk
Of the 102,869 women who took part in the study, 2,059 of them were later diagnosed with invasive breast cancer. The researchers examined the confounding risk factors related to cancer occurrence, but they found no overall link with night shift work.
The team also did not find any significant difference in breast cancer risk related to the type of night shift work the participants had, the age at which they started working, or whether they began before or after their first pregnancy.
The only statistically important trend that the researchers found was associated with average night hours the participants worked every week. However, Jones and his team said this result was not supported by earlier evidence or even by any proposed biological explanation.
Breast Cancer Now CEO Delyth Morgan said they hope that the results of their study will help many women who work night shifts feel more assured that their job patterns are not going to increase their risk for the disease.
While the researchers may not have found any correlation between shift work and breast cancer risk, they warned that such working habits may still produce adverse effects on people's health.
The findings of the Breast Cancer Now-sponsored study are featured in the British Journal of Cancer.
