The human body constantly releases an extremely faint form of light known as ultraweak photon emission. Its wavelengths can fall within the visible spectrum, but the glow is roughly 1,000 times dimmer than the naked eye can detect. In other words, the light is technically visible, yet its intensity is not.
Where the light comes from
This glow is not the infrared radiation we feel as body heat. It appears to arise largely from ordinary metabolic chemistry. When cells use oxygen and process energy, reactive molecules can form. Reactions involving these molecules may leave other molecules in excited states; as they settle, tiny packets of light called photons are released.
Researchers captured this phenomenon with highly sensitive, cooled cameras in a dark room. Their human-body imaging study found that emission changed through the day, reaching its highest measured level in the late afternoon. The face also produced more detectable light than several other areas.
What this does—and does not—mean
You will not see someone glowing by switching off the lights, even after your eyes adjust. Detecting the emission requires specialized equipment that can register extraordinarily small numbers of photons. It is also different from the bright bioluminescence of fireflies, which relies on specialized chemical systems.
Scientists are interested in whether these emissions might eventually help monitor metabolism, oxidative stress, skin condition, or disease without invasive procedures. That possibility remains experimental, however; a faint glow is not currently a diagnostic tool, aura, or sign of anything supernatural.
The effect is another reminder that our bodies are chemical systems, not static objects. From this hidden shimmer to the thousands of miles of blood vessels carrying oxygen through us, much of human biology operates beyond perception. We glow continuously; we simply lack the sensory equipment to notice.