Human brains emit light and glow in the dark, revealing our mental state

All living tissues give off photons as excited molecules shed excess energy. The phenomenon is so subtle – roughly a million times dimmer than the threshold of human vision – that researchers call it ultra-weak photon emission (UPE).

Because the brain consumes an exceptionally high metabolic budget and is packed with photoactive compounds such as flavins and serotonin, scientists have long suspected it might glow more brightly than other organs.

What they did not know was whether that ghostly glow fluctuates in ways that reveal real-time neural activity.

Turning darkness into a laboratory

To find out, a collaborative team from Algoma University, Tufts University, and Wilfrid Laurier University recruited 20 healthy adults and seated them in a light-sealed chamber.

Two photomultiplier tubes – devices capable of counting single photons – were aimed at the back (occipital) and side (temporal) regions of each participant’s head, while a third tube monitored background darkness.

Simultaneously, the researchers fitted the volunteers with a standard electroencephalography (EEG) cap to compare electrical rhythms with any optical rhythms.

The experiment lasted ten minutes and cycled through five simple conditions: eyes open, eyes closed, listening to a repetitive tone, eyes closed again, and finally, eyes open again.

Each condition lasted two minutes – enough time for the researchers to watch the brain light settle into a steady signal before switching tasks.

Brain glow follows a rhythm

Despite the photons’ extreme faintness, the team, led by Hayley Casey and Nirosha Murugan, successfully separated brain-derived light from background counts by examining complexity and variability.

In brief, the cranial signal showed higher entropy – richer fluctuation – than the near-constant dark noise inside the optics.

Spectral analysis revealed that these emissions waxed and waned at very slow frequencies below one hertz. This means a rise-and-fall cycle occurred every one to ten seconds.

This low-frequency “heartbeat” was strongest over the occipital cortex, the very region that processes visual input.

Just as important, the photon stream reached a reproducible equilibrium within each two-minute block, then shifted when the task changed. That pattern strongly suggests the glow is not random metabolic chatter but reflects the brain’s moment-to-moment state.

Eyes shut, photons rise

Closing one’s eyes is known to boost alpha-band electrical rhythms (8–12 hertz) in the visual cortex. This is a signature of relaxed wakefulness first reported in the 1920s.

The new study revealed that occipital photon counts modulated in tandem. Some participants’ emissions rose during eyes-closed periods, while others fell. However, each individual showed a consistent direction across the two eyes-closed blocks.

Occipital UPE also showed modest correlations with alpha power measured by EEG – evidence that the optical signal does tap into neural dynamics. However, the relationship is complex and needs further investigation.

Auditory stimulation, delivered through a simple repeating tone, produced subtler effects. The temporal sensor picked up changes in photon variability that tracked certain EEG rhythms recorded over the same region.

This hints that metabolic shifts linked to processing sound also leave a photonic fingerprint.

A window into brain metabolism

Standard brain-monitoring tools, whether MRI, PET, or infrared spectroscopy, rely on sending energy into the skull and measuring what bounces back.

UPE recording, in contrast, is wholly passive: the detectors simply wait in the dark. That makes the technique conceptually similar to EEG or magnetoencephalography (MEG), but tuned to oxidative chemistry rather than electricity or magnetism.

The authors propose “photoencephalography” for a future technology that could use natural brain light as a clinical signal.

“We view the current results as a proof-of-concept demonstration that patterns of human-brain-derived UPE signals can be discriminated from background light signals despite very low relative signal intensity,” explained the researchers.

They speculate that refined detectors – equipped with wavelength-specific filters to isolate, say, green-emitting flavins from red-shifted lipofuscin – might one day diagnose metabolic stress.

Such tools could also monitor aging or even flag neurodegenerative disease before symptoms emerge.

Future research directions

The pilot study sampled only 20 people and monitored just two scalp sites. Photomultiplier tubes counted all visible and near-infrared photons, drowning out any subtle color coding of brain chemistry in the noise.

Future setups will deploy arrays of narrow-band detectors to capture spatial and spectral detail, and will compare cranial readings with simultaneous measurements from non-neural tissues to confirm specificity.

The researchers also plan to test broader age ranges and clinical populations. They aim to see whether disorders that alter oxidative metabolism – such as Alzheimer’s disease, traumatic brain injury, or chronic stress – produce distinctive optical signatures.

Why the brain glows at all

Though focused on measurement, the study builds on work linking UPE spikes to bursts of reactive oxygen in mitochondria. Those by-products can excite biomolecules; when the molecules relax, they release photons.

If so, the faint light may function as a built-in reporter of the brain’s redox balance – a chemical ledger of energy production, stress, and recovery.

Electrical rhythms revolutionized neuroscience a century ago; functional MRI did the same in the 1990s.

Photoencephalography, though still in its infancy, offers a tantalizing third window. It peers directly at the cell’s metabolic engine without injecting energy or contrast agents.

The current study lights the first path forward: prove the signal exists, show that it changes with mental state, and establish that it differs from background noise.

The next challenge is to turn those wisps of light into a practical tool for medicine and cognitive science.

The study is published in the journal iScience.

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