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If you live in the developed world, darkness can be hard to find. Our plugged-in, 24-7 lifestyles deliver a barrage of not just technology, but also light. All the time. And this may have negative health consequences that scientists are only just beginning to understand.

We are a diurnal species, which means we are genetically programmed to do our work during the day when it's light out, and sleep when it's dark. For many thousands of generations as hunter/gathers we did just that, with the only light after dark coming from the moon, stars or firelight. With the industrial revolution and the invention of electric light, all that changed.

We went from incandescent electric light to fluorescents and neon. Now we are increasing our use of solid-state lighting, such as LEDs (light-emitting diodes).

"There is a tremendous energy savings to be realized as you replace incandescent and fluorescent light sources with LEDs, but then the question is what are the health concerns," said George Brainard, a professor at Thomas Jefferson University's Jefferson Medical College and director of its Light Research Program.

We already know there are health impacts from light. For starters, light can be used therapeutically; the most well-known cases are the treatment of winter depression or seasonal affective disorder.

"Lighting stimulus is neither good or bad, it's how it's used, what it's used for and when it is used," said Brainard. "But just like a drug, any drug that can heal, can also cause harm. If light has the capacity to invoke a therapeutic benefit, it also has the flip side: it can have unwanted side effects when used inappropriately."

To understand how light impacts our health, you have to understand a little about how the eye works. "The eye mediates two different effects of light," said Alfred J. Lewy of Oregon Health and Science University. "One effect of course is vision - any of the visible wavelengths hitting rods or cones mediates the vision."

But the other effect has to do with another part of our visual system that operates below our level of consciousness - these are photoreceptors in our eyes that regulate biology and behavior. "More specifically, the circadian system, our biological rhythms, our neuroendocrine system (that's hormonal), and neurobehavioral responses," said Brainard.

Scientists learned that bright light could suppress the production of the hormone melatonin (which helps regulate our circadian rhythms). They learned that this photoreceptor system is especially sensitive to shorter wavelengths, which is light that appears blue to us.

This discovery tweaked how light therapy could be done. Instead of blasting patients with high-intensity light from the entire spectrum, they could use less intense light that came from the part of the spectrum we are most sensitive to: blue light. It was a therapeutic win.

But what happens if we are subjected to light at night, particularly light that contains a lot of blue to which we are most sensitive? We already know that despite being diurnal creatures we don't go to sleep the moment it gets dark. When the sun sets, we turn on the lights, and we often also turn on other devices like TVs, computers, tablets, and electronic readers.

"What if that light is enriched with blue wavelengths that are sending a signal to your circadian system, your neuroendocrine system, that it is full-bore daytime and you want your body in top alertness?" asked Brainard. "When you shut off lights to go to sleep in the evening, is it going to delay and possibly disrupt sleep during the night?"

The answer is... we're still trying to figure that out. The scientific "literature is young," says Brainard.

But here's what we do know.

A 2012 report from the American Medical Association Council on Science and the Public Health confirmed that our 24-hour light-dark cycle is important for our circadian rhythms, the central nervous system, as well as many biological processes, including the release of melatonin. But, "Pervasive use of nighttime lighting disrupts these endogenous processes and creates potentially harmful health effects and/or hazardous situations with varying degrees of harm," the AMA report stated.

If disruption of sleep and natural cycles becomes chronic it can have far-reaching effects and a range of disorders. Long-term shift workers (who sleep on average nine hours less per week than daytime workers) and are subjected to light during the entire night have more than twice the occurrence of cardiovascular disorders compared to daytime workers, said Brainard. Of course, that doesn't mean that their health problems are definitely caused by less sleep or more light; there are likely a variety of factors, said Brainard, but "long-term loss of sleep or interruption of sleep and of the circadian systems are associated with very significant health consequences."

And that may be related to cancer. The AMA reported,
"Limited epidemiological studies support the hypothesis that nighttime lighting and/or repetitive disruption of circadian rhythms increases cancer risk; most attention in this arena has been devoted to breast cancer."
Writing for the New York Times, Laura Beil explained,
"Light at night has been examined as a contributor to breast cancer for two decades. While there is still no consensus, enough laboratory and epidemiological studies have supported the idea that in 2007, the World Health Organization declared shift work a probable carcinogen. Body clock disruptions 'can alter sleep-activity patterns, suppress melatonin production and disregulate genes involved in tumor development,' the agency concluded."
But even if you're not a shift worker you may be exposed to more light at night than you realize, with electronic devices glowing in your bedroom and street lamps outside the window. A study done in Israel found, "Women living in neighborhoods where it was bright enough to read a book outside at midnight had a 73 percent higher risk of developing breast cancer than those residing in areas with the least outdoor artificial lighting," wrote Ron Chepesiuk in Environmental Health Perspectives.

It's not just outside light that's a factor. What happens inside our homes is important, too. We are entertaining ourselves late into the evening with our electronic toys. A survey found that 95 percent of Americans said they used an electronic device (computer, TV, cell phone, video game) within an hour of going to sleep at least several times a week. Sixty percent said they had some kind of sleep problem just about every night.

Maybe we're too stressed out, or we drink too much coffee, or maybe some of us are being impacted adversely by light at night. When it comes to the light we're most sensitive to, blue-rich light, we are getting even more of it because many of our electronic screens have blue-enriched LED systems. There has been an increase in the use of LEDs in homes, businesses and municipalities (even streetlights) to save money and increase efficiency, and many of these lights are "cool white" lights that are blue-rich.

"We can think of children and teens who like to play games on their devices until they are forced to go to be bed," said Brainard. "In many of those cases they may be getting an inadvertent dose of bright, blue-enriched light through a computer-type device."

And that could actually cause your melatonin to drop. A study from the Lighting Research Center at Rensselaer Polytechnic Institute found that "a two-hour exposure to electronic devices with self-luminous 'backlit' displays causes melatonin suppression, which might lead to delayed bedtimes, especially in teens." Two hours of using such a device resulted in a 22 percent drop in melatonin, which could affect sleep said one of the study's authors.

A Harvard study found that when subjects were exposed to 6.5 hours of blue light versus green light, the "blue light suppressed melatonin for about twice as long as the green light and shifted circadian rhythms by twice as much (3 hours vs. 1.5 hours)."

Handheld electronic devices could pose a greater risk than other light because "the closer your eye is, the greater the intensity," said Lewy. "If you decrease your distance by half you increase your intensity by a factor of four." If you're holding your phone close to your face to play a game, it will have a much greater effect than if you're sitting five feet away from your TV.

And it's not just humans who are affected. Animals experience the same effects on physiology from light - and the same sensitivity to blue wavelengths. It affects their circadian rhythms, their annual rhythms and seasonality, said Travis Longcore, the science director for the Urban Wildlands Group. It's a problem that dates back to the proliferation of electric light.

"There are certainly instances in the literature that go back to the early parts of the 1900s showing that birds in London were laying eggs when they shouldn't when they were next to newfangled electric street lights," Longcore said. Now we know that light affects everything from turtles to frogs to birds to a whole host of mammals. As more light is used outdoors that is blue-rich, we may see greater impacts.

So what do we do?

What most researchers have concluded is that we need to do more research. But already we have seen some helpful information.

"There are some studies out now that have shown that reducing the blue content of lighting in the evening has less of a disrupting effect on the onset of hormone secretion and other biological and behavioral effects," said Brainard. One study found that wearing amber lenses at night, which block blue wavelengths, resulted in an improvement of sleep quality and improved mood.

There are companies that sell products such as goggles that block blue light or filters that can be put over your computer screens that will do the same. How well they work in the long-term remains to be seen. Another company, f.lux has a program you can download to your computer that will shift the screen's colors depending on the time of day.

"Our thoughts are that you want brighter, blue-shifted light in the morning and regular good old-fashioned white light during the day, and you want red-shifted, dimmer light in the evening," said Brainard. He is helping to design lighting for the astronauts at the space station and he believes we may see some of that technology and design elements trickle down to those of us on Earth.

"I believe that in the fullness of time, decades out, all of architectural lighting will change and change dramatically within our lifetimes, because the traditional values didn't take into account the biological-behavior consequences of lighting," said Brainard. "Now when I attend meetings with the Department of Energy and the lighting industry, it is increasingly appreciated that future designs must factor in the health consequences of lighting."

When it comes to outdoor lighting and the effect on humans and wildlife, Longcore believes we're going to get smarter about the kind of light we use and how we use it. Already, he says, manufacturers are developing lights that don't have blue in them, which have been used around observatories.

Using LEDs may seem problematic to some now, but only because we started off using ones that were blue-rich. "In the future, outdoor lighting is all about control," said Longcore. "Because when you switch to LEDs without a penalty in energy or expense, you can turn them up or down. You can use bi-level or tri-level adjustable lighting."

For example, imagine a parking lot that has bright lights during the parts of the evening it's most heavily trafficked, say until 11pm. After that the intensity of the lights could be turned down to a half or a quarter, allowing a motion sensor to turn it back up if someone walks by in the wee hours of the night. "That is the future of energy efficiency and reducing the environmental impacts of outdoor lighting," said Longcore.

"The Dutch already do this in environmentally sensitive areas - they use 'bat lamps,' street lights that are very yellow to red, so they won't attract insects and allow bats to move across and not be diverted," he said. "But they also use street lights that go off at a certain hour and they put little tracer lights halfway up the pole, so people know where the road is going but it doesn't illuminate the whole thing. When you switch from overhead to tracer lights people drive slightly closer to the middle of the road but there is no change in the number of accidents."

The key to how we employ light all comes down to duration, spectrum, intensity, timing, and need. "Don't light if you don't need to; use motion-sensitive light; longer wavelengths are better; you don't need as bright a light as engineers usually say and people generally prefer less light; have it illuminated for the amount of time you need and no longer; and direct it where you need it and nowhere else," Longcore advised.

Fortunately for us, the technology is already moving in that direction and that will help us reduce our impact on wildlife, regain some of our night skies, and maybe even improve our own health.

About the author

Tara Lohan is a freelance writer and former senior editor at AlterNet. She is the editor of two books on the global water crisis, including Water Matters: Why We Need to Act Now to Save Our Most Critical Resource.