Women who live in areas with higher levels of outdoor light at night may be at higher risk for breast cancer than those living in areas with lower levels, according to a Harvard University study.
The large long-term study also found a stronger association among women who worked night shifts. “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,” said lead author Peter James, an assistant professor at Harvard Medical School.
The study, published in the journal Environmental Health Perspectives, looked at data from nearly 110,000 women enrolled in the Nurses’ Health Study II from 1989 to 2013. The researchers linked data from satellite images of Earth taken at night-time to residential addresses for each study participant, and also considered the influence of night shift work. The study also factored in detailed information on a variety of health and socioeconomic factors among participants.
Women exposed to the highest levels of outdoor light at night – those in the top fifth – had an estimated 14 per cent increased risk of breast cancer during the study period, as compared with women in the bottom fifth of exposure, the researchers found. As levels of outdoor light at night increased, so did breast cancer rates.
The link was stronger among women who worked night shifts, suggesting that exposure to light at night and night shift work contribute jointly to breast cancer risk.
Other research by Harvard scientists based on the same Nurses’ Health Study II had earlier found an association between shiftwork and breast cancer.
- Peter James and others. Outdoor light at night and breast cancer incidence in the Nurses’ Health Study II, Environmental Health Perspectives, volume 125, issue 8, 17 August 2017. doi: 10.1289/EHP935