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. 

 

In several studies conducted over the past decade, scientists have found that increased exposure to light at night correlates to higher cancer rates, particularly for those cancers like breast or prostate that require hormones to grow. Women who work the night shift have been shown to have higher risks of breast cancer, while blind women have lower-than-average risk. In 2009, a landmark study by epidemiologist Dr. Richard Stevens, of the University of Connecticut, compared cancer rates and the presence of light at night across 164 countries and found that women in industrialized, highly lit countries had a 30 to 50 percent higher risk of breast cancer than those in countries with less light pollution. 

 

That last study would be contentious had Stevens not been researching breast cancer for decades, and had he not already studied most of the other elements that might explain such a difference between developed and less developed countries. “Back in the 80s, we were all convinced that it had to be the high-fat Western diet that was causing the spike in breast cancer rates, but then the really good dietary models started to come in and nothing proved it out,” Stevens says. “Then we thought, ‘okay, maybe it has to do with the average age of the first child,’ because that typically gets older as societies get richer, but that wasn’t it either. We looked at a couple of other things that typically change when a society becomes more developed as well, but we’ve looked carefully at all of them, and those changes cannot account for the vast majority of the increase. So what else could it be? That’s how I got into this light at night deal.”

 

Stevens authored some of the first studies linking light at night (called LAN by scientists) to cancer in the late 1990s and early 2000s and since then the evidence has continued to mount. Light pollution has now been linked to a host of mood disorders as well, and even to obesity and diabetes. The International Agency for Research on Cancer (IARC, part of the World Health Organization) officially deemed shift work a “carcinogen” in 2009. There is, of course, more research to be done, but given the evidence out there already Stevens calls light pollution “the worst thing we’ve done to ourselves and the planet since global warming.” 

 

The good news is that light pollution is a relatively new problem and it’s entirely reversible, starting with ensuring nighttime darkness at home wherever possible. According to Joan Roberts, Ph.D., a Fordham University professor whose work on the importance of natural light in maintaining circadian rhythms also touches on the need for darkness, “It’s not that you need total darkness, but you need to have NO stray light that is in the blue or green region (400-600 nm) in the late evening.”

 

The blue/green light is what is emitted from TVs , computers, clock radios and other gadgets (cell phones, Blackberry's, etc) or from what Roberts calls “light trespass,” which comes in from the outside, through your windows. “Light from the blue region stimulates a ‘wake up’ circadian response in the body and blocks the production of melatonin in the brain.” 

 

The same thing happens if you get up in the middle of the night to go to the bathroom and turn the light on. Stevens recommends using a red night light instead, because red light does not suppress melatonin production. Unfortunately, popping a few melatonin tablets won’t solve the problem, and in fact it could actually exacerbate it. Researchers have found that creating an artificial spike in melatonin makes the circadian disruption worse. 

 

“Another thing: If you wake up in the night, stay in the dark,” Stevens says. “It’s completely normal to awaken during the night. They used to say if it happened, you should get out of bed and go watch TV or read a book. That is completely wrong. We’ve completely lost sight of so many things in modern life – that period of quiet wakefulness in the dark … we evolved with that.”

 

A version of this story originally appeared on GOOD. Read it here.