You may be surprised to learn that the human brain, often studied under the lens of its electrical and chemical activity, also releases light. It’s a very faint, invisible light that has sparked a great deal of speculation across the decades since its discovery.

Now, for the first time, scientists have measured ultraweak photon emissions (UPEs) — the minuscule bursts of light generated by the brain — directly from outside the skull. The findings suggest the brain doesn’t just think. In a way, it shines.

“The very first finding is that photons are coming out of the head — full stop,” said Nirosha Murugan, a biophysicist at Wilfrid Laurier University and senior author of the study. “It’s independent, it’s not spurious, it’s not random.”

What those photons mean is still unclear, but the study suggests a possible link between the brain’s metabolism and its optical signature. This endogenous glow may serve as a new kind of non-invasive signal, potentially revealing physiological or cognitive states through what researchers are calling “photoencephalography.” In other words, “brain light” may encode information in ways never previously considered.

A Different Kind of Brain Signal

The brain’s glow is not like bioluminescence — the firefly-like radiance created by special enzymes. Nor is it the infrared thermal radiation from body heat. Instead, it is thought to be the result of spontaneous photon release from biological metabolism.

This light is unimaginably dim — millions of times weaker than what the human eye can see. Yet, it occurs constantly in all living tissues. For nearly a century, scientists have speculated whether there’s more to these ultraweak emissions than meets the eye. They may represent a new mode of biological signaling.

The idea dates back to the 1920s, when the Russian scientist Alexander Gurwitsch proposed that dividing cells emit “mitogenetic rays,” which he believed could influence the behavior of nearby tissues. In the 1970s and 1980s, the German physicist Fritz-Albert Popp coined the term “biophotons” and argued (highly controversially) that these emissions might synchronize cellular processes across the body.

Despite such bold claims, most of the scientific community regarded biophotons as noise: a byproduct of molecular excitation, not a functional signal.

But Murugan and her colleagues weren’t chasing an old theory. They were interested in something new. Could these photons be used to monitor the brain, perhaps even track cognitive states, without sending any energy into it? In other words, could light coming out of the brain serve as a window into what’s happening inside?

Capturing Light from the Thinking Brain

To find out, the researchers designed a sensitive experiment using photomultiplier tubes. These are devices capable of counting individual photons. They placed these sensors just above two regions of the skull: the occipital lobes, which process visual information, and the temporal lobes, which handle sound. A third sensor, pointed away from the head, captured background light.

Participants sat quietly in total darkness and completed a series of simple tasks: resting with eyes open or closed, or listening to rhythmic clicking sounds. Meanwhile, the scientists recorded photon counts from each detector, along with brainwave activity via EEG.

What they found surprised them.

Not only did the photon detectors register a clear, consistent signal from the brain — but the signal changed with the tasks. The photons pulsed in slow rhythms, typically between once every 10 seconds and once per second, and the spectral characteristics and entropy of these pulses were distinct from background light.

The researchers dubbed the method “photoencephalography.”

Light and Brainwaves: A Weak Link?

But while the photon emissions were clearly real, their relationship to brain activity remained puzzling.

The brain is a metabolically hungry organ, using about 20 percent of the body’s energy despite being just 2 percent of its mass. When neurons fire, they ramp up oxidative reactions that can release excited molecules — potential sources of UPEs.

This led researchers to suspect that photon counts might rise during more active mental states. And, in some ways, they did.

For example, when participants closed their eyes (which typically boosts alpha brain waves), UPEs from the occipital lobe also changed. There was even a correlation between the variability of photon emissions and the spectral power of EEG signals during these periods.

But the patterns weren’t always clear or consistent. Some expected correlations between UPEs and brainwaves failed to appear. In other cases, light detected over one region of the brain correlated with activity in a different, distant region. Maybe some photons travel, scatter, or are absorbed in complex ways within the brain, but scientists just don’t know yet.

What Could These Photons Be Doing?

The study’s data leave room for multiple interpretations.

One possibility is that these photons are just metabolic exhaust, the byproducts of oxidative stress and normal brain activity. Their frequency, variability, and spectral features may still provide useful biomarkers, even if the photons don’t carry information themselves.

Another possibility, more speculative, is that the photons do transmit information within the brain. Some researchers have suggested that certain neurons, especially myelinated axons, might act as optical waveguides, similarly to light-transmitting fibers like those in fiber optic cables.

If that’s true, the brain might be using light to supplement or modulate its traditional electrochemical signals. So far, that remains a hypothesis without direct evidence. But subtle signs, including light sensitivity in unexpected brain molecules like serotonin and flavins, keep the door open.

Still, even if the photons serve no purpose beyond indicating brain metabolism, the implications are significant. A future generation of photoencephalography devices might detect early signs of neurodegeneration, metabolic imbalance, or even trauma, based on patterns of faint light flickering from within the head.

For now, technical hurdles remain. The detectors used in the study could measure total photon counts but couldn’t distinguish between different wavelengths. This parameter is key if brain light is to be linked to aging, cognitive potential, and disease.

Future experiments will need denser sensor arrays, better spatial resolution, and selective filters that can identify specific spectral fingerprints. The goal is to turn faint glows into precise maps of brain function.

And then there’s the biggest unknown: What causes the photon emissions to change in the first place?

In the end, it may turn out that we are not just electrical or chemical beings. We may also be faintly luminous ones.

The findings appeared in the journal iScience.