Even if you lock yourself in the darkest room imaginable, something will still emit photons: your own skin. It is not heat or infrared radiation, but light within the visible spectrum. The problem is that it is so faint that your eyes will never be able to register it.
The question seems simple: if the body emits visible light, why don’t we see ourselves glow? The answer lies in a paradox of scale. The signal exists, is measurable and has a technical name, ultra-weak photon emissionbut its intensity remains well below the threshold of human vision. Laboratory instruments are needed to capture it, and it is advisable to separate from the beginning what science has measured from what is still speculation.
A light that is not heat
The first thing is to undo a common confusion. Any body at body temperature emits infrared radiation, the same radiation that thermal cameras detect. This radiation is not visible and responds to heat. The ultra-weak emission of photons is something else: light that falls, at least in part, within the range that a human eye could theoretically perceive, between blue and red.
The difference is not anecdotal. Infrared heat comes out of all objects simply because they have a temperature. The emission in question arises from specific chemical reactions within the cells, and Its presence says something about what happens in the metabolism, not just about how many degrees the skin has..
Chemistry that goes out in the form of a photon
The accepted mechanism is a form of chemiluminescence, the same general principle by which certain reactions release light instead of heat. In the body, the origin is in cellular oxidation. The so-called reactive oxygen species, very unstable molecules that metabolism generates naturally, react with lipids, proteins and nucleic acids.
High-energy intermediates emerge from these collisions. Some leave molecules in an electronically excited state, like a compressed spring building up tension. When that molecule returns to its stable state, it releases the excess energy, and a fraction escapes in the form of a photon. The biology of this process links it to the activity of mitochondria, the organelles where fuel is burned to obtain energy and where a good part of these reactive species are produced.
The most honest image is not that of a body that “lights up”, but that of a luminous trace: the detector registers the microscopic flash that each reaction leaves when it goes out. There is nothing to indicate that these photons carry a message. They are, as far as we know, a byproduct of oxidative chemistry. But measuring them forced us to build devices capable of counting light particles one by one.
The figures that put the phenomenon on a scale
The first serious quantitative reference came in 1990. The team of R. Edwards and his collaborators recorded on human skin between 170 and 600 photons per second per square centimeterwith variations depending on the area of the body. To give you an idea, one photon per second is a ridiculous amount of light compared to the billions that enter the eye when you look at a screen. Hence the signal is invisible no matter how much we insist on darkness.
The leap came in 2009. Masaki Kobayashi, Daisuke Kikuchi and Hitoshi Okamura photographed the body’s spontaneous emission with a cooled and extremely sensitive CCD camera, the type of sensor used in astronomy to capture very dim objects. They worked with five volunteers at different times and observed a pattern: the emission was lowest in the morning, rose throughout the day and reached a maximum around 16:00.
That same work measured a local peak close to 3,000 photons per second per square centimeter on the cheek. It should be emphasized that this figure corresponds to a specific point, not to the average emission of the entire organization. The light is not distributed equally.
What color does a finger shine?
Just because the signal is measurable doesn’t mean we know everything about it. It was necessary to know its color, that is, its spectrum. In 2016, Masaki Kobayashi, Torai Iwasa and Mika Tada analyzed a fingertip with a sensitive camera attached to a diffraction grating, an element that separates light into its wavelengths like a prism would.
Emission detected between 450 and 750 nanometersthat is, from blue to red, with the highest intensity concentrated between 570 and 670 nanometers, in the yellow and red range. It’s an elegant measurement, but it has an important limit: it describes one finger. It does not prove that the entire body surface emits the same color distribution, because the tissue, temperature, metabolic state and the conditions of the experiment themselves can modify the signal.
What the light still doesn’t tell us
Here it is advisable to brake. It’s tempting to jump from these measurements to ambitious conclusions, and that’s where rigor makes the difference. A review published in 2023 compiles the state of the art and describes ultra-weak emission as a possible indirect indicator of oxidative processes, with a general order of magnitude of between 10 and 1,000 photons per second and per square centimeter in biological systems. The same review proposes it as a line of research for future non-invasive tools, not as a validated clinical test.
The distinction is crucial. The fact that there is a correlation between emission and metabolism does not prove that this light allows us to diagnose a disease on its own, nor to distinguish it from other factors that also alter cellular oxidation. There is also no evidence that photons serve to communicate information between cells, no matter how attractive the idea is. And the 16:00 maximum, measured in just five people, is a clue to a possible connection with daily rhythms, not a law that governs the entire population.
It is worth insisting on the vocabulary, because it brings errors. Talking about “human bioluminescence” suggests luminous organs like those of a firefly, and that is not what happens. There is no aura, no perceptible glow and no known function. There is chemistry that is turned off by releasing photons, and much of the scientific dissemination on the subject stumbles precisely at that point, turning a weak signal into a mystical story.
The real frontier of this field is not in proving that we shine, that has already been measured, but in finding out what reliable information that light hides and how to capture it without confusing it with the noise of the instruments or the environment. As long as there is no clear threshold that separates a genuine metabolic variation from a measurement artifact, the question will remain open: the body emits photons, yes, but we still do not know how to read what they say.
