New study indicates link between artificial light and breast cancer
Published: 11 Sep 2017

A study conducted between 1989 and 2013 suggests women who are exposed to large amounts of artificial light could be 14% more likely to develop breast cancer, with those working night shifts at highest risk.

The study, the Nurses' Health Study II, was conducted by researchers from Harvard university's TH Chan School of Public Health who analysed data from 109, 672 women, making it the largest study into breast cancer and night shifts.

A large number of lifestyle risk factors were taken into consideration including the working hours of female nurses and the home addresses of each study participant were linked with data from satellite images of earth taken at night.

The study, published on 17 August 2017, found breast cancer rates increased as participant's exposure to outdoor light at night increased. There was an estimated 14% increased risk of breast cancer in the top fifth of exposure compared with the lowest fifth of exposure.

The association between outdoor light at night and breast cancer was found only among women who had not yet reached menopause, and those who were past or current smokers. The link between light at night and breast cancer risk was also more prominent among women who had worked night shifts.

The study gave the following conclusion:

"This prospective study, conducted over 22 year of follow-up with time-varying and objective measures of ambient LAN (light at night) across the contiguous United States, provides evidence that women living in areas with high levels of outdoor LAN may be at higher risk of breast cancer even after accounting for individual and area-level risk factors for breast cancer. Although further work is required to confirm our results and to clarify potential mechanisms, our findings suggest that exposure to outdoor light at night may contribute to breast cancer risk."

Lead author of the study, Peter James summarised:

"In our modern industrialised society, artificial lighting is nearly ubiquitous. Our results suggest that this widespread exposure to outdoor lights during night time hours could represent a novel risk factor for breast cancer."

The researchers admitted the study had limitations such as possible exposure misclassification due to missing satellite data and data processing errors, but also argued it had many strengths such as the longevity of the study.

Earlier studies have indicated that melatonin, a hormone produced in the human brain, appears to suppress the growth of breast cancer tumours, but exposure to light may decrease its levels, disrupting circadian rhythms and, in turn, lead to increased breast cancer risk.

The link between breast cancer and night shift work has been widely debated and in 2007 the International Agency for Research on Cancer which is part of the World Health Organization classified night shift work as a "probable carcinogen".

Since then, however, a study commissioned by the Health and Safety Executive from October 2016 found that night shift work had no or little effect on breast cancer.