Women who work in night shifts may be at an increased risk of developing breast cancer, as artificial light blocks a hormone that plays a key role in suppressing growth of its tumours, claims a study.

The study, published in journal Environmental Health Perspectives, showed that women who were premenopausal and were current or past smokers, and also who live in areas with high levels of outdoor light at night, were at risk of developing breast cancer - the most common cancer in women worldwide."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, Assistant Professor at Harvard University.elatonin, a hormone produced in the human brain, appears to suppress the growth of breast cancer tumours, but exposure to light may decrease its levels, thus disrupting circadian rhythms -- our internal "clocks" that govern sleepiness and alertness -- and, in turn, lead to increased breast cancer risk, James said, citing previous studies.

For the new study, the team looked at data from nearly 110,000 women and linked data from satellite images of Earth taken at night to residential addresses for each study participant, as well as considering the influence of night shift work. The results showed that 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, as compared with women in the bottom fifth of exposure.As levels of outdoor light at night increased, so did breast cancer rates, the researchers said, but acknowledged that further work is required to confirm the study findings and clarify potential mechanisms.