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A study at RPI’s Lighting Research Center tested the effects of digital tablets and different colors of light on subjects’ circadian rhythms.Brittany Wood et al. / RPI

In the summer of 2012, the American Medical Association (AMA) adopted a new policy stating that nighttime light exposure is hazardous to human health. “The primary human concerns with nighttime lighting include disability glare [the reduction of visibility caused by bright light, and a major driving risk]… and various health effects,” a summary of the AMA’s policy (pdf) read. “Among the latter are potential carcinogenic effects related to melatonin suppression, especially breast cancer.” Even low levels of light, the report said, could suppress the production of melatonin, a hormone that’s secreted at night, which signals for our bodies to sleep and can also suppress the growth of tumors.

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There is reason to find this a little alarming. Well past sunset, many of us are still plugging away at our mobile devices and computers, sometimes working into the wee hours of the night. Some habitually fall asleep to the flickering glow of the television or simply keep the bulbs burning through the night. Artificial light makes it possible for us to work and play at any hour, and it can also serve a comforting presence as we drift off to sleep. (See the article “Drowning in Light,” by Dirk Hanson, for more on the centuries-long boom in artificial lighting.) So it’s disconcerting to hear that something as seemingly harmless as light could actually put us at risk of getting cancer.

“Light and dark are equally as important to our health,” says Mark Rea, director of the Lighting Research Center* at the Rensselaer Polytechnic Institute. “We’ve done enough research that shows that human beings are really Jekylls and Hydes. So when it gets dark and we become Mr. Hydes, we really need to be asleep.”

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When we’re exposed to light, whether it’s coming from the Sun or an electric bulb, it interacts with a system of photoreceptors, specialized photosensitive neurons in the retinas of our eyes. There are three kinds of photoreceptors: rods that help us identify moving shapes in the dark, cones that help us see colors, and, in fewer numbers, ganglion cells, which only recently became known in the past decade. The latter have a direct connection to the brain, specifically the suprachiasmatic nucleus (SCN), a rice-grain–sized region in the hypothalamus that controls our circadian rhythms, the approximately-24-hour daily cycle of our bodies. The SCN also sends signals to the pineal gland, which secretes the hormone melatonin, but researchers are still trying to figure out how this communication works and how it influences hormone production.

For light to stimulate the SCN, it has to be at least 10 times as intense as the minimum amount of light we need to see. So a dim bedroom reading lamp probably isn’t strong enough to shake up your circadian clock, according to Rea. But there are common sources of light that can activate the SCN. In 2012 Rea and other researchers published a study to determine how light from self-luminous tablets, such as iPads, could affect melatonin levels of young, college-age adults. They found that two hours of exposure from back-lit tablets prior to bedtime could suppress melatonin levels by 23 percent and potentially hamper a person’s ability to sleep. But spending one hour with the tablet didn’t significantly affect melatonin.

“An irregular relationship with light can expose the frailties we are born with.”

Research suggests that it’s not only the quantity of light, but also the quality.  Tablets are one of many light sources that emit blue, or short-wavelength, light. Rea and his colleagues conducted a study released in 2009 that showed that both red (long-wavelength) and blue light could promote alertness at night, but only the blue light dragged down melatonin levels.

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Other studies indicate that blue light could also affect our moods. Last August scientists at Ohio State University examined hamsters for “depressive behaviors” when they were exposed at nightlights of varying wavelengths. (Moods were measured by how much sugar water and how much regular water the hamsters drank; drinking sugar water is an activity that hamsters enjoy, so if they do it less, the researchers took that as an indication that they were getting less enjoyment in general.) The study found that the shorter the light’s wavelength, the less sugar water the hamsters drank. These findings could potentially be applied to humans, according to the study’s lead author Tracy Bedrosian. She also noted that compact fluorescent light bulbs have a higher level of blue light than incandescent light bulbs, most of which are being phased out in the U.S.

Most alarmingly, some research in recent years has suggested that nighttime light exposure could be linked to breast cancer, including an influential study published in The Lancet on night-shift nurses. This study prompted the World Health Organization in 2007 to release a report saying that night-shift work is a “probable carcinogen,” and was partially responsible for the AMA’s declaration that nighttime light is a health hazard. Linking long-term light exposure to cancer, however, is complicated. While researchers don’t deny that receiving light at odd hours could potentially perpetuate something like breast cancer, the cause-and-effect relationship is uncertain and indirect. Light, as Rea comprehends it, is not an agent that causes disease, but could increase the risk of developing conditions you have genetic predispositions to, such as breast cancer or heart disease. “An irregular relationship with light can expose the frailties we are born with,” he says.

Rea’s laboratory is currently conducting numerous studies on light exposure in humans, using small devices called Daysimeters (a play on “dosimeter,” a device used to measure a person’s exposure to radioactivity). He and his researchers attach the Daysimeters to their subjects, which include nurses, teachers, and those who have disordered circadian rhythms, such as Alzheimer’s patients who tend to be awake during the night. The device collects data on the subject’s light exposure, which then informs clinical treatments to help correct their circadian clocks.

Scientists like Rea are hoping to discover the mechanisms that explain how the brain detects light and use that knowledge as a basis for improving lighting technology and our use of it. In the meantime, it seems that one of the best ways to keep from running off the clock is a habit our parents have long nagged us about: stick to a regular bedtime. 

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* Correction: The article originally misstated the name of the Lighting Research Center. 

Eli Chen is a freelance journalist based in Brooklyn who has contributed to The New York Times, Science Friday’s blog, and OnEarth magazine. She is also a production associate at the World Science Festival. 

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