There are plenty of arguments against lighting the night sky: It wastes energy, blots out stars and messes with the nocturnal habits of animals in a big way. Now there’s another reason, one that could go a long way toward convincing humans that whatever sense of safety is conferred by nighttime lighting, it isn’t worth the risk. It turns out that light pollution may be a cancer risk.
That statement is bold, but increasingly the science is proving it out. Humans, as well as many animals and plants, need regular exposure to darkness to maintain what’s called the circadian rhythm—essentially the body’s internal clock, which governs various bodily functions. Of particular interest to researchers is the fact that darkness at night tells the body to produce certain hormones, most importantly melatonin, which not only aids sleep, but also helps to maintain immune system function.