Most people spend a few hours per day looking at screens.

In fact, breaking down the time you spend staring at LEDs can be downright depressing; between laptops, television sets, and smartphones, the number can climb quickly. According to one study, the average smartphone user looks at their device more than 1,500 times per week.

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All of that screen time can have an effect on your body. In recent years, blue light, the visible wavelengths of blue put out by our electronic devices, has been the subject of significant scrutiny. Dozens of smartphone apps claim to cut blue light, and many newer phones have built-in settings to limit blue light exposure.

That prompts an obvious question: what does blue light really do?

According to the “Harvard Health Letter,” the extra illumination affects our biological clocks.

Circadian rhythms, the mechanism our bodies use to tell time and function appropriately, rely in part on light. Blue wavelengths are naturally present during the day, and our bodies associate them with sunlight.

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When our eyes receive blue wavelength light during the daytime, our bodies react by boosting reaction times, attention span, and mood. That’s helpful during the day, but not so ideal at night; blue light also suppresses melatonin, a hormone that controls sleep cycles.

In short, LED lights disrupt sleep, leading to a host of potential issues.

Some studies indicate that the health risks aren’t limited to a few hours of missed sleep. People who work night shifts regularly often develop work shift sleep disorder, characterized by insomnia, depression, heart disease, diabetes, and obesity. There’s even a tenuous link between light exposure and certain types of cancer, although the Harvard Health Letter notes that they still must fully explore this link before it’s considered clinically actionable.

Light in the blue wavelengths is particularly effective at decreasing melatonin levels. In fact, blue light is thought to affect melatonin levels for twice as long as green wavelength light.

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“In terms of light and our brains, there is a spectrum of wavelengths that impacts the human circadian system,” David Earnest, professor and circadian rhythms expert at the Texas A&M Health Science Center College of Medicine, said to Live Science. “Blue light is the most sensitive side of the spectrum.”

Blue light apps work by limiting exposure to these potentially problematic wavelengths.

Activate an app, and you’ll notice that your smartphone’s screen seems to dim. The app applies a filter that prevents the most harmful wavelengths—but not all of the wavelengths. Experts strongly recommend ditching the smartphone at least 30 to 60 minutes before you head to bed.

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“Ideally, you want your environment to be dimly lit so your body can start naturally producing melatonin,” said Dr. Robert Oexman, the director of the Sleep to Live Institute.

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For those of us that fall asleep with our phones, that’s a difficult proposition. Plus, we can’t simply switch over to a laptop or television since all LEDs have the same basic effect. Changing bedtime habits can be difficult, but your long-term health is well worth the effort.