Scientists have found yet another reason for you to get some sun this winter

Worth venturing out into the cold.
Worth venturing out into the cold.
Image: Reuters/Jim Young
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It’s winter in the northern hemisphere. As temperatures fall, a lot of us are pulling out our winter coats and getting ready to hunker down inside. But as tempting as it is to avoid the cold, it’s probably worth spending time outside. Scientists just found another hidden benefit of getting out of the house: a boost in immune cells triggered by a specific type of sunlight.

Researchers from Georgetown University Medical Center found blue light—light with a short wavelength that comes from both the sun and, for example, the lit-up screens of devices like smartphones—helps circulate a type of immune cells in our skin. When these cells, called T-cells, flow around the body more effectively, they’re able to reach new infections more quickly, and stop them before they actually cause harm. The Georgetown work was published in Scientific Reports on Dec. 20.

Skin is the body’s largest organ. It’s primary role is to safeguard all of our organs by acting as a physical barrier. Most obviously, it keeps microbes that may cause infections out of our bloodstream and protects us from exposure to the elements. And it’s also responsive to the external environment: When exposed to ultraviolet (UV) light from the sun, for example, the skin cells that make melanin, produce more of the pigment to protect us from damaging radiation. UV light also triggers the production of vitamin D, which helps us absorb nutrients like calcium and iron and can signal to certain immune cells that it’s time to get to work to fight off an infection.

It turns out another blue light produced by the sun can set off a different immune response. Blue light is on the shorter end of the wavelength of light we can see (not as short as UV rays, which are invisible to us). Because its wavelengths are so small, blue light can make it deeper into our skin, where T-cells are. To test the impact of blue light, the research team took samples of mouse and human T-cells and exposed them to blue light.

In low levels like the kind we’d get outside, it turns out the blue light triggered T-cells produce hydrogen peroxide, which gives them a chemical kick in the pants, forcing them to circulate more effectively. These swirling T-cells are like guards patrolling the perimeter of a secure area: The more they circulate around, the more chances they have to spot any potential microbial threats, and eliminate them before they do any damage.

The research team so far has only used cell models, rather than seeing the system work in a living subject. But they know human skin has an abundance of T-cells, and expect that on a larger scale, blue light from the sun does help these cells circulate throughout the body.

Of course, sunlight isn’t all great. Overexposure to its  UV radiation can lead to skin cancer, which is why lathering up on sunscreen is important—even in the winter.

Luckily, natural blue light doesn’t come with the same risks, and may even make you feel more awake and happier. The blue light that comes from smartphone and computer screens, though, is “short-wave enriched,” meaning it’s at a higher concentration than what we’d get outside. This concentration can throw off your natural sleep cycle if you get too much of it at night.