Invisible to the Naked Eye, Our Brains Emit Light — and We Are Learning How to Decode It

In a discovery that sounds like science fiction, researchers have shown that human brains emit flashes of light that can pass through the skull. These signals are so tiny and faint that they’re a million times dimmer than what we can see, but new research shows they might carry important clues about brain function.

In a study published in iScience, scientists from Algoma University and Wilfrid Laurier University in Ontario, along with collaborators at Tufts University in Massachusetts, set out to understand whether ultra-weak photon emissions, or UPEs, could be used to track mental states. Their findings open the door to a new, entirely passive way of monitoring brain activity.

What Is Brain Light?

UPEs are tiny amounts of light given off by living tissues, something researchers have known about for decades. First described in 1923, these faint glimmers are produced when excited molecules return to a resting state, often as a byproduct of metabolic processes like energy use and oxidative stress. While all parts of the body emit some UPEs, the brain gives off more than most because of its intense activity and dense concentration of light-sensitive molecules.

Until recently, this phenomenon was more of a curiosity than a practical tool. But that’s starting to change. Scientists have found that UPEs seem to shift with changes in our mental state, fluctuating with sleep-wake cycles, hormone levels, even cognitive effort. The wavelengths of this light can also vary with age and brain health. Researchers now suspect that UPEs may not just be byproducts of brain activity but could actually play a role in how cells communicate.

Despite growing interest, UPEs have rarely been used in practice because of how difficult they are to detect. The light is incredibly faint and easily drowned out by other signals, so it wasn’t clear whether it could be measured reliably or used to infer anything meaningful about brain function.

Read More: Brain Processes Information Like Ocean Waves

Measuring the Brain’s Glow

To find out, the research team recruited 20 healthy adults and placed them in a completely dark room. Using sensitive photodetectors called photomultiplier tubes, they measured light emissions coming from two regions of the head: the back, where the brain processes visual information, and the sides, which handle sound. Additionally, participants wore EEG (electroencephalography) caps to track their brain’s electrical activity.

Over 10 minutes, participants were asked to close and open their eyes and listen to simple sounds, tasks known to affect brain rhythms. The researchers found that the UPEs weren’t just random static. The emissions followed slow, rhythmic patterns and shifted in predictable ways during different tasks. For instance, UPEs changed noticeably when people opened or closed their eyes, aligning with well-known changes in brain wave activity.

Importantly, these light emissions were not influenced by any external stimulus; they were entirely natural, passive signals coming from the brain itself.

New Technology to Measure Brain Activity

The idea of using UPEs to monitor the brain has an intriguing advantage over current imaging tools. PET scans, fMRI, and even near-infrared spectroscopy (fNIRS) involve applying energy to the brain, which can in some cases influence the very activity researchers are trying to study.

In contrast, tools like EEG and magnetoencephalography (MEG) passively record electrical or magnetic signals without affecting the brain. UPEs could offer a similar, light-based alternative.

While the findings are promising, the researchers emphasize that this is just the beginning. More sensitive detectors and filters will be needed to tease apart different wavelengths of brain light. Eventually, machine learning could help interpret these patterns and even detect signs of brain disorders.

For now, it’s a remarkable reminder: Our brains don’t just think — they shine.

Read More: Brain Scan AI System Can Describe What You See

Article Sources

Our writers at Discovermagazine.com use peer-reviewed studies and high-quality sources for our articles, and our editors review for scientific accuracy and editorial standards. Review the sources used below for this article:

• iScience: Exploring ultraweak photon emissions as optical markers of brain activity

• Mayo Clinic: Electroencephalogram

Having worked as a biomedical research assistant in labs across three countries, Jenny excels at translating complex scientific concepts – ranging from medical breakthroughs and pharmacological discoveries to the latest in nutrition – into engaging, accessible content. Her interests extend to topics such as human evolution, psychology, and quirky animal stories. When she’s not immersed in a popular science book, you’ll find her catching waves or cruising around Vancouver Island on her longboard.