Why would hormone-sensitive cancers like those of the breast and prostate be affected by light at night, but not lung or colon cancers? It can be explained by the primary mechanism believed to mediate the relationship between ALAN and adverse health defects. Melatonin, the hormone whose name is derived from the Greek word melas (black, dark), is secreted from the brain in all animals in anticipation of nightfall, to entrain circadian rhythms — unless, that is, the retina is exposed to light, which results in the shutdown whistle getting blown at the old melatonin factory.
David Blask, a colleague of Hill’s at Tulane who runs the Laboratory of Chrono-Neuroendocrine Oncology there, has been studying the effects of melatonin on cancer for almost 40 years. One of his latest studies, published in 2015, found that exposure to bright blue-white light during the day has the opposite effect of light exposure at night: It greatly enhances the release of melatonin and inhibits the proliferation of prostate cancer cells in an animal model.
“We evolved to see bright blue, full-spectrum light during the day and to have complete blackness at night,” Blask says. “Both are really healthy for your circadian system. It’s all about this oscillatory balance under natural conditions between light and dark.”
In a 2005 study, he described what happened when he took melatonin-depleted blood from premenopausal women who had been exposed to light at night and infused it through human breast cancer xenografts in nude rats. The cancer, he found, grew faster.
But cancer is far from the only adverse effect thought to be associated with exposure to light at night. Randy Nelson, professor and chairman of neuroscience at Ohio State University Wexner Medical Center, has published more than two dozen studies done in mice and hamsters showing that light at night increases body weight and anxiety-like behaviors. Satellite studies of outdoor lighting have also found an increased risk of obesity in humans who live in highly lit areas. Hill has co-authored a paper suggesting that the reduced melatonin expression researchers had already associated with aging, and the accompanying loss of circadian rhythms, may underlie the increased risk of virtually all diseases in older adults.
Tired of hearing what light at night may or may not do, younger scientists are clamoring for greater precision — beyond the laboratory setting — in measuring light exposure, melatonin levels and their relationship to disease states.
Perhaps for no other reason than that we’re up and about, eating and drinking, at a time when we should be sleeping.
“You’ve got to know what your quantities are,” says Mariana Figueiro, a professor at the Lighting Research Center at Rensselaer Polytechnic Institute in Troy, N.Y. Speaking in March at a conference on circadian disruption by the National Institute of Environmental Health Sciences, Figueiro called on the field to step up its game. “Until you know the amount of light you’re talking about, the dose, you can’t really say much about the effects. Some researchers will say, ‘Oh my God, if you open up the refrigerator at night you’re going to die from that light.’ ”
Based on her dozens of studies in the field of chronobiology, Figueiro says she finds it implausible that dim light coming through a window shade, or from under a bedroom door, could possibly be bright enough to suppress melatonin — particularly when a person’s eyes are closed during sleep.
“If you’re living in an area with more street light, you’re likely to have more of a night life,” she says. “You probably stay up later, eat later, drink later. That doesn’t mean that the streetlight itself is causing cancer.”
In response to the studies that link levels of outdoor light at night to cancer risk, she published a study measuring light levels at bedroom windows, inside bedrooms and at the corneas of female schoolteachers, finding no consistent link to the satellite-measured levels of outdoor light.
Another researcher seeking to bring increased rigor to the field is Eva Schernhammer, associate professor in the Department of Epidemiology at the Harvard School of Public Health. Despite having published three dozen studies on the hormone, she says, “we still do not know enough about melatonin. We have imperfect measures for doing research on its effects. If you take a morning urinary measure of melatonin, for instance, that gives you only the average level during the entire night.”
Although Italian researcher Paolo Lissoni has published studies suggesting that melatonin supplementation appears to improve cancer survival, Schernhammer says, “the problem with his papers is they haven’t really randomized people properly. They leave you with a bit of a question mark.”
To find out if avoiding light at night truly reduces disease risk, or if an evening dose of melatonin has the same benefit, large and properly randomized trials are necessary. But to convince funding agencies that such trials are warranted, Schernhammer says, existing data from smaller studies first need to be pooled and analyzed.
“We submitted a proposal to pool worldwide data on melatonin and breast cancer risk,” she says. “A pooled analysis would allow us to either put the melatonin story to rest or to have the data necessary to support a large randomized trial. But the National Cancer Institute wasn’t interested.”
For now, about all that researchers are certain of is that artificial light at night distorts our circadian rhythms and is associated with all sorts of bad health outcomes, perhaps for no other reason than that we’re up and about, eating and drinking, at a time when we should be sleeping.
Whether a lifestyle in remote Madagascar, enjoying bright light by day and deep darkness by night, will directly result in reduced risk of cancer, obesity, depression and more remains unproven.
But it sure will make for a suprachiasmatic view of the Milky Way.
[This article originally appeared in print as "Let There Be Dark."]