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Screens May Be Terrible for You, and Now We Know Why

Stashed in: Sleep!, Light, Blue Light

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The problem is blue light.

A fast-growing body of research has linked artificial light exposure to disruptions in circadian rhythms, the light-triggered releases of hormones that regulate bodily function. Circadian disruption has in turn been linked to a host of health problems, from cancer to diabetes, obesity and depression. “Everything changed with electricity. Now we can have bright light in the middle of night. And that changes our circadian physiology almost immediately,” says Richard Stevens, a cancer epidemiologist at the University of Connecticut. “What we don’t know, and what so many people are interested in, are the effects of having that light chronically.”

Stevens, one of the field’s most prominent researchers, reviews the literature on light exposure and human health the latestPhilosophical Transactions of the Royal Society B. The new article comes nearly two decades after Stevens first sounded the alarm about light exposure possibly causing harm; writing in 1996, he saidthe evidence was “sparse but provocative.” Since then, nighttime light has become even more ubiquitous: an estimated 95 percent of Americans regularly use screens shortly before going to sleep, and incandescent bulbs have been mostly replaced by LED and compact fluorescent lights that emit light in potentially more problematic wavelengths. Meanwhile, the scientific evidence is still provocative, but no longer sparse.

As Stevens says in the new article, researchers now know that increased nighttime light exposure tracks with increased rates ofbreast cancerobesity and depression. Correlation isn’t causation, of course, and it’s easy to imagine all the ways researchers might mistake those findings. The easy availability of electric lighting almost certainly tracks with various disease-causing factors: bad diets, sedentary lifestyles, exposure to they array of chemicals that come along with modernity. Oil refineries and aluminum smelters, to be hyperbolic, also blaze with light at night.

Yet biology at least supports some of the correlations. The circadian system synchronizes physiological function—from digestion to body temperature, cell repair and immune system activity—with a 24-hour cycle of light and dark. Even photosynthetic bacteria thought to resemble Earth’s earliest life forms have circadian rhythms. Despite its ubiquity, though, scientists discovered only in the last decadewhat triggers circadian activity in mammals: specialized cells in the retina, the light-sensing part of the eye, rather than conveying visual detail from eye to brain, simply signal the presence or absence of light. Activity in these cells sets off a reaction that calibrates clocks in every cell and tissue in a body. Now, these cells are especially sensitive to blue wavelengths—like those in a daytime sky.

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