The invisible light the human body emits

HostIf you go into a room with no windows and click the light switch off, you probably think you're standing in total darkness. But it turns out you're actually standing in a room filled with visible light. It's just that the light is coming from you. Your skin is shedding tiny bits of light like a dim, biological ember. Why is it that we can't see this glow ourselves?

GuestWell, for a long time, people just suspected it was happening. They thought all living things might leak a little light as a side effect of being alive. But nobody could prove it until 2009. A team in Japan used these incredibly sensitive cameras. To make them work without the cameras picking up their own noise, they had to cool them down to about a hundred and twenty degrees below zero. They took a group of volunteers and put them in total darkness for days at a time. And the cameras actually caught it. They got the first real images of the human body glowing. And the wildest part is that this isn't just heat. We have known for a long time that our bodies give off heat, which you can see with night-vision goggles. But this was actual visible light. We're talking about colors like green, yellow, and red coming right off our skin at a level that's roughly a thousand times lower than what our eyes can pick up.

HostWait, so if I have a camera that's cold enough, I can see myself in the dark?

GuestYou really can. To give you an idea of how dim we're, think about the very smallest, dimmest star you can see in the night sky. The light coming off your forehead is thousands of times weaker than that. Even though it's there, we're essentially blind to our own shine because the intensity is just so low.

HostSo where's the light actually coming from? Is it like a firefly?

GuestNo, and that's what makes it so interesting. It isn't like a firefly or a fish at the bottom of the ocean. Those animals have specialized organs made just to make light. For us, the glow is more like a persistent chemical accident. Our bodies are powered by reacting with oxygen to create energy. As we process that oxygen, we produce these very reactive molecules called free radicals. These molecules are essentially the sparks from our internal engine. The glow happens when those free radicals run into the fats and proteins in our cells. That collision excites the electrons, and when they release that extra energy, it comes out as a tiny flash of light. Scientists call this ultra-weak photon emission. Since your face is out in the sun more and has less covering, those chemical reactions happen more often there. That makes the face the brightest part of the body, especially the spots around your mouth and your cheeks.

HostI'm having a hard time with the idea that my cheeks are brighter than my arms just because they see more sun. If I'm in a dark room for days, wouldn't that sun effect wear off?

GuestIt's less about the sun hitting you in that moment and more about the state of the skin itself being more active. And even in total darkness, that light isn't steady. One of the biggest surprises in the research was that our glow follows a strict daily rhythm. It rises and falls like a tide based on our body clock. It hits its peak in the late afternoon and then hits the bottom late at night. It's a direct mirror of how fast our bodies are burning fuel. When your internal fire is cranked up during the day to support your activity, the chemical crashes that make the light happen much more often. Researchers are actually looking at whether we could use this light as a way to check a person’s health or stress levels without ever having to draw blood. We could just measure the light.

HostBut if we're constantly leaking this light, it feels like a waste that we can't see it. Why didn't we evolve to use it?

GuestIt comes down to how much noise is in the system. The human eye needs about a hundred bits of light hitting it at once to register a signal. But our skin only lets off a handful of those bits from any one spot every second. If our eyes were sensitive enough to pick up that tiny, tiny bit of light, we would be in big trouble. We would be living in a constant, glowing fog of our own making. Imagine trying to see a predator creeping through the shadows of a forest if your own arms and legs were constantly shining like lanterns right in front of your eyes. You would never be able to see the dim movements of anything else because your own body's light would be blinding you.

GuestWe had to stay blind to our own biological light so we could actually see the world around us.

HostThose dark, windowless rooms aren't actually empty then, because we're standing there in a cloud of our own green and red light, even if we need to stay in the dark to see anything else.