Japanese researchers in 2009 placed five men individually inside a dark room and captured a faint glow from their bodies, 1,000 times too weak for human eyes to see; but strangely, the invisible light wasn't constant: it grew stronger through the day and peaked around 4 p.m.

In a completely dark room in Japan, five young men sat alone, one at a time. Their bodies had been wiped clean. They had spent 15 minutes letting their eyes and bodies adjust to the darkness. Then, sitting quietly in a chair, each volunteer faced a camera for 20 minutes.

But this was no ordinary camera. The device was sensitive enough to detect extraordinarily tiny amounts of light, down to the level needed to capture the faint photons naturally emitted from the human body. The room was completely light-tight. The camera itself was cooled to −120°C to reduce interference.

And what the researchers saw was something invisible to the human eye. The human body was emitting light. Not enough for another person standing nearby to see. Not enough to make anyone glow in the dark like a science-fiction character. The light was roughly 1,000 times weaker than the naked human eye's sensitivity.


But to the camera, it was there. Even more surprisingly, the glow did not remain constant throughout the day. It changed with time. The human body appeared to glimmer in rhythm with its internal clock.

The experiment that photographed something humans cannot see

The remarkable experiment was described in a 2009 research paper titled “Imaging of Ultraweak Spontaneous Photon Emission from Human Body Displaying Diurnal Rhythm,” published in PLOS ONE on July 16, 2009. The study was led by Masaki Kobayashi, alongside Daisuke Kikuchi and Hitoshi Okamura. The researchers recruited five healthy male volunteers in their 20s and followed them over several days.

The men maintained normal light-dark cycles and slept on a regular schedule. On the days of the experiment, they were brought into a specially designed darkroom at different times of the day.

Then came the unusual part. Each participant was placed in complete darkness. After a period of dark adaptation, the upper half of his body was photographed continuously for 20 minutes using an extremely sensitive charge-coupled device, or CCD, camera.

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Measurements were taken every three hours from 10 a.m. to 10 p.m., and the process continued for three days. The researchers also measured body temperature and collected saliva to analyze cortisol, a hormone known to follow a daily rhythm.

What emerged from those photographs was evidence of something humans carry with them all the time but can never see with their own eyes. A faint, ultraweak emission of photons.

Yes, the human body literally emits light

The phenomenon is known as ultraweak photon emission, sometimes referred to as biophoton emission. It is different from the visible bioluminescence produced by organisms such as fireflies. Fireflies generate visible light through specialized biological reactions. The light detected in the human body, however, is extraordinarily faint and is thought to arise as a by-product of ordinary biochemical and metabolic processes.

The researchers explained that virtually all living organisms can spontaneously emit extremely weak light without external photoexcitation. In humans, this emission is associated with biochemical reactions involving energy metabolism and reactive oxygen species.

The study's abstract put the discovery in strikingly simple terms: “The human body literally glimmers.”

But this glimmer is far too faint for human vision. That is why the researchers needed a highly specialized imaging system capable of detecting the tiny number of photons emitted from the body's surface.

The face appeared brighter than the rest of the body

When the researchers analyzed the images, they found that the photon emission was not evenly distributed.

The face emitted more light than the body. And even the face was not uniform. The areas around the mouth and cheeks showed stronger emission than the sides of the face and the eye sockets. The upper body also showed changes over time.

At its strongest, the emission from the cheeks reached around 3,000 photons per second per square centimeter at about 4 p.m., roughly double the level measured at 10 a.m., according to the researchers.

But the most intriguing discovery was not simply where the body emitted photons. It was when.

The invisible glow followed a daily rhythm

The researchers found that photon emission was relatively weak in the morning. It increased during the day. And it reached its highest point in the late afternoon, around 4 p.m.

Afterward, the emission gradually declined.

The pattern was sufficiently consistent for the researchers to conclude that the human body exhibited a ‘diurnal rhythm’ in photon emission.

To test whether the pattern was simply due to changes in lighting conditions, the researchers conducted an additional experiment. Three volunteers were kept awake in a constant light environment, and their photon emissions were measured during the early hours of the morning.

The emissions remained low between 1 a.m. and 7 a.m., even under constant light conditions. That suggested the rhythm was not simply a reaction to the outside environment.

Instead, it appeared to be linked to something inside the body. Something governed by the body's own biological clock.

It was not body heat

One possible explanation might seem obvious. Could the glowing pattern simply be related to temperature? The researchers tested that possibility. They compared the ultraweak photon images with thermographic images showing body surface temperature.

But the two patterns did not match. Areas producing strong photon emissions were not necessarily the warmest areas of the body.

The researchers found no significant correlation between daily photon intensity and body temperature, suggesting that the changing glow was not simply a consequence of temperature or blood circulation near the skin.

Instead, the answer appeared to lie deeper, in the chemistry happening continuously inside living cells.

What could be making the human body glow?

The researchers proposed that the photons originate from processes connected to energy metabolism. As cells generate energy, biochemical reactions can produce reactive oxygen species and free radicals. These can react with molecules such as lipids and proteins, creating electronically excited molecules.

When those excited molecules return to a lower-energy state, they can release tiny amounts of energy as photons. The result is an extraordinarily faint biological glow.

The researchers wrote that the mechanism is thought to originate from the generation of free radicals during energy metabolic processes, with subsequent biochemical reactions ultimately leading to photon emission.

In other words, the invisible light may be a faint physical trace of the chemical activity constantly taking place inside us.

Every cell is working. Molecules are reacting. Energy is being transformed.

And some of those processes may leave behind photons too faint for our eyes to notice.

The body's glow may also be connected to its internal clock

The researchers also examined cortisol, a hormone that follows a strong daily rhythm. They found that cortisol levels generally decreased from morning toward evening, while photon emission increased.

Across the volunteers, cortisol showed a negative correlation with photon emission intensity.

This did not mean that cortisol itself was necessarily causing the body to glow.

But the relationship suggested a broader possibility: that ultraweak photon emission was linked to physiological processes regulated by the body's circadian system.

The researchers suggested that changes in cellular metabolism regulated by the circadian clock could help explain why the intensity of the invisible glow rose and fell over the course of a day.

Their final conclusion was almost poetic. Through the chemical processes of cellular metabolism, they suggested, the human body “glitters to the rhythm of the circadian clock.”

So, are humans really glowing right now?

Technically, yes. But there is an important distinction. This is not the kind of glow portrayed in movies, paranormal stories or science fiction. You cannot see another person shining in a dark room, and the study does not suggest that humans produce visible light. The emission is extraordinarily weak, about 1,000 times below the naked eye's sensitivity.