A large, long-term study from Harvard T.H Chan School of Public Health, shows a strong correlation between women who live in areas with high levels of outdoor lighting at night and increased risk of breast cancer.

The study is the most comprehensive and large to date, looking at data from almost 110,000 women registered to the Nurses’ Health Study II (NHS) from 1989-2013. The study surveyed a considerable number of lifestyle risk factors, such as working hours of female nurses. Researchers used satellite images of Earth taken at night to link residential addresses for each study participant. Women working nightshifts are particularly at risk. Women with the top fifth level of exposure had an estimated 14% increased risk of breast cancer as compared to women in the bottom fifth of exposure. The study found, as levels of exposure to outdoor light at night increase, so do the rates of breast cancer.

Artificial outdoor lighting emits blue wavelength light. Blue wavelength light, such as that emitted from the screens of electronics and energy-efficient lighting, not only disrupts sleep but may contribute to the causation of cancer, diabetes, heart disease and obesity. It is not known why exposure to night-time light is harmful, however, exposure to light suppresses melatonin secretion, which affects the “circadian rhythm”, i.e. the internal clocks that determine alertness and sleepiness in living beings. Experimental evidence links lower melatonin levels to the association with cancer.

Peter James, an assistant professor at Harvard Medical School’s Department of Population Medicine and the lead author of the study, said: “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.”