Do humans glow in the dark?

We know people give off heat. Thermal cameras turn us into glowing orange blobs on screen. But that is infrared, invisible to our eyes, and not real light in the everyday sense. So surely humans do not actually glow? It turns out we do, in genuine visible light. It is just far too faint for anyone to notice.

The short answer

Yes. In 2009, researchers Masaki Kobayashi, Daisuke Kikuchi and Hitoshi Okamura, of the Tohoku Institute of Technology and Kyoto University in Japan, photographed the human body emitting tiny amounts of visible light. They reported that the glow is about 1,000 times weaker than the sensitivity of the naked eye. Their study was published in the journal PLoS ONE.

The real story

To catch the glow, the team needed extraordinary equipment and complete darkness. They used a highly sensitive, cryogenically cooled camera, chilled to around minus 120 degrees Celsius and capable of detecting single photons, the smallest possible units of light. Five healthy male volunteers were imaged in a light-tight room for 20 minutes every three hours, over three days.

The images revealed that the light rises and falls through the day. The glow was brightest in the late afternoon, around 4 pm, and weakest in the morning, around 10 am. The face glowed more than the rest of the body, especially the cheeks. The researchers suggested the daily rhythm could be linked to the body’s internal clock and changes in metabolism.

So where does the light come from? It appears to be a side effect of simply being alive. As your cells turn food into energy, they produce highly reactive molecules called free radicals. These react with fats and proteins in the skin, creating excited molecules called fluorophores. When those molecules settle back down, they can release a photon of light. It is a faint chemical glow, a very distant cousin of the light from a firefly, and far less dramatic.

Crucially, this is not the same as body heat. Thermal cameras detect infrared radiation, which every warm object gives off. The light measured in this study is in the visible part of the spectrum, the same kind of light our eyes evolved to see. We are just not built to see anything quite this dim.

Why people get it wrong

Most people’s only experience of seeing human bodies in the dark comes from thermal imaging, in films, documentaries and the occasional search-and-rescue clip. That trains us to think the only glow we produce is heat. Meanwhile, real bioluminescence is something we associate with jellyfish, glow-worms and fireflies, not ourselves. The truth sits in between: humans do emit visible light, but at a level so feeble that it takes a camera chilled to minus 120 degrees to notice.

The Sole’s verdict: “You do glow, mate. Just a thousand times too faintly for anyone to notice. Which, honestly, is how most of us feel by 4 pm.”