Your body gives off visible light, just too faint to see

3 min read

Every person gives off a small amount of visible light. It is not heat or infrared radiation, but real photons in the range our eyes can in principle detect. The catch is that the glow is roughly a thousand times too weak for anyone to see, so it took an ultra-sensitive camera and a pitch-black room to capture it.

What the researchers recorded

In 2009, Masaki Kobayashi of the Tohoku Institute of Technology in Sendai and colleagues, including circadian biologist Hitoshi Okamura of Kyoto University, published images of this light in the journal PLoS ONE. They worked with five healthy male volunteers in their twenties who had no skin conditions and were not taking oral medication.

Each volunteer sat bare-chested in a light-tight room in complete darkness. A camera able to register individual photons photographed them for 20 minutes at a time, every three hours from 10 a.m. to 10 p.m., over three days. The pictures showed a faint but genuine emission from the skin, strongest across the face.

Where the light comes from

The glow is a by-product of ordinary metabolism. As cells use energy, they produce reactive molecules known as free radicals. These react with fats and proteins in the skin and create molecules in an excited, high-energy state. When that energy is handed on to light-emitting molecules called fluorophores, some of it escapes as photons. The study measured this light at wavelengths between roughly 500 and 700 nanometres, which spans green through red in the visible spectrum.

The face glowed more than the body. The cheeks were brightest, followed by the upper neck and the forehead. The authors suggested that the extra exposure of the face to sunlight, and the presence of melanin, which contains fluorescent components, might boost the emission there. They presented that as a possibility rather than a proven cause.

A glow that follows the clock

The most unexpected result was that the light changed with the time of day. Emission was at its lowest at 10 a.m., climbed through the afternoon and peaked around 4 p.m., before gradually falling again into the evening. At the cheeks, the reading at 4 p.m. was about twice the 10 a.m. value.

That timing pointed to a link with the body’s internal clock. The researchers also collected saliva samples and found that the light was negatively correlated with cortisol, a hormone that follows a strong daily cycle. When cortisol was higher, emission tended to be lower. Because metabolism itself rises and falls over the day, a rhythm in its faint light output makes biological sense.

Why such a weak signal is useful

Making the glow visible required serious engineering. The team used a CCD camera cooled to minus 120 degrees Celsius to cut electronic noise, together with a custom lens system, so that a handful of photons would stand out against the background. Under normal conditions, any light in a room overwhelms the signal completely, and even in darkness the human eye is far too insensitive to register it.

Researchers are interested in this light because it reflects the chemistry happening inside living tissue. Free radicals are involved in ageing and in many disease processes, so a method that tracks their activity without touching the body could one day help monitor health. For now, the main takeaway is simpler. Living tissue is never truly dark. Even in a sealed room, every person is quietly shining, just below the edge of perception.