Breast Cancer and Night Light

Uninterrupted hours of nighttime darkness encourage healthy melatonin levels in the blood that dramatically suppress the growth of breast tumors. On the other hand, exposure to light at night causes melatonin levels in the blood to plummet, stimulating growth of breast cancer cells – according to a groundbreaking study appearing in the December 1, 2005 Cancer Research. Funding for this study came from the National Cancer Institute (NCI) and the National Institute of Environmental Health Sciences (NIEHS). The study could help explain why breast cancer is up to five times more common in industrialized nations than in underdeveloped countries (nearly half of all breast cancers cannot be explained by currently accepted risk factors). It could also help explain why night shift workers have higher rates of breast cancer, and why the blind have lower rates. We already knew that melatonin is a powerful protective substance that the body produces at night during the dark. We knew that light exposure at night can rob us of melatonin. But this study is the first to solidly connect light exposure at night to human cancer. The authors studied human breast cancers that had been grafted into rats. The rats were infused with blood collected from healthy women. Some of the blood was collected at night during the darkness (and was rich in melatonin); some of the blood was collected at the same time of night, but following 90 minutes of exposure to white light. The melatonin-rich blood collected in darkness powerfully suppressed the tumors; the blood from women exposed to light at night stimulated the tumors, and increased the tumors fat uptake. This also held true in other rats with their own liver cancers.

Over the past 100 years, increasing numbers of people have been exposed to more artificial light after sunset, both at home and in the workplace. This study suggests that minimizing nighttime light exposure, strengthening the body’s own circadian rhythm, and encouraging normal melatonin secretion could help reverse rising cancer rates. Taking steps to support the circadian rhythm makes good sense, based on what we now know. (There is a whole section on simple, proven methods to strengthen the circadian rhythm for improving sleep in my book, From First Kicks to First Steps).

The current study also suggests new areas for research – such as types of nighttime lighting that don’t suppress melatonin for those who need light at night, and types of daytime lighting that better mimic the value of natural light. Changing the eating habits of night shift workers might also prove to make a difference (they tend to eat more high fat foods at the same time their melatonin is suppressed).

But in the short run, I encourage parents to help their children learn to sleep with as little light as possible, and to begin turning down bright lights after sunset, where practical, to give your children the fullest benefit of their own remarkable melatonin.

Alan Greene MD FAAP