The human body gives off a faint glow of visible light, produced by ordinary chemical reactions inside living cells. It is real light, but it is far too weak to see: roughly a thousand times below the sensitivity of the naked eye, so to us the body appears completely dark.

The human body gives off a faint glow of visible light, produced by ordinary chemical reactions inside living cells. It is real light, in the range our eyes are built to see. It is also far too weak for us to notice, running at roughly a thousand times below the sensitivity of the naked eye, which is why, to each other, we appear completely dark.

The glow has been photographed. In 2009, a team led by Masaki Kobayashi at the Tohoku Institute of Technology used a cooled, single-photon-sensitive camera in a sealed dark room to image healthy volunteers, and reported the results in PLOS ONE. The body glowed, the face most of all, and the brightness rose and fell across the day, peaking in the late afternoon.

What the glow actually is

The phenomenon has a deliberately unglamorous name: ultraweak photon emission, sometimes called biophoton emission. It has been observed in every living system examined, from bacteria and plants to animals and people, at an intensity of roughly ten to a thousand photons per square centimetre each second, across wavelengths from the ultraviolet through the visible to the near-infrared.

Its source is ordinary metabolism. Oxidative reactions in cells, particularly those involving reactive oxygen species, leave molecules in a briefly excited state, and when those molecules settle back down they shed the extra energy as a photon. The light is a byproduct of the chemistry of staying alive, not the output of any organ built to make light.

That last point separates it from bioluminescence. A firefly produces bright, visible light through a dedicated enzyme system evolved for the purpose. The human glow is the opposite: faint, incidental, and far below the threshold of sight.

Why it is not body heat

This is the part that is easiest to get wrong. The body does emit a great deal of electromagnetic radiation because it is warm, but that radiation is thermal infrared, sitting well outside the visible range. That is what a thermal camera detects, and it is separate from the ultraweak visible photons measured in these experiments. Ultraweak photon emission is different: much fainter, partly in the visible range, and apparently coming from cellular chemistry rather than temperature.

The evidence that it is not simply heat is direct. Kobayashi’s team found that the photon-emission pattern did not match the body’s thermal image and was not significantly correlated with body temperature. And when a group at the University of Calgary imaged living and freshly dead mice in 2025, publishing in the Journal of Physical Chemistry Letters, the live mice glowed far more than the dead ones held at the same body temperature. Heat alone cannot explain that difference. Metabolism can.

A signature of being alive

That live-versus-dead contrast points to what the glow tracks. It is not a constant. It rises and falls with the day in humans, it climbs in plants that are injured or heat-stressed, and it drops when the metabolism producing it stops. The emission is, in effect, a faint optical readout of oxidative activity in the tissue.

That is why there is genuine research interest in it. If the light reports on oxidative stress, then measuring it might offer a non-invasive way to gauge the state of skin or other tissue, and a few groups are exploring exactly that. It is worth being clear that this is an early research direction rather than an established tool, and the sensitive cameras required are not yet anything found in a clinic.

Where the science stops

The faint glow also attracts claims that run well past the evidence, and the two need keeping apart. That living cells emit ultraweak light as a product of oxidative metabolism is established and measurable. Whether that light also carries useful biological information is a real and open question, and some researchers are investigating possible signalling roles and medical uses, in a field that is still developing.

What is not established is the stronger version, associated with the term biophoton, in which cells supposedly communicate through a coherent light field tied to health, consciousness or a visible aura. That idea remains speculative, and its more elaborate forms are not science at all.

The honest description is the plainer one. The body emits a real but extremely faint visible light, generated by the same oxidative chemistry that keeps cells running, weak enough that we never see it and distinct from the heat we radiate. What remains genuinely open is what, if anything, that light can tell us, not whether it is there.