Runner’s High: How Running Affects the Brain. It’s Not Just Endorphins

For decades, the runner’s high was dismissed as either a myth or a mere placebo effect. It wasn’t until brain imaging using PET scans revealed the truth behind what happens after a long-distance run. This is the story of a breakthrough that fundamentally changed our understanding of the passion for running.

The Myth of “Happy Hormones” – Early Experiments and Dead Ends

n the world of 1970s sports medicine, a term was born that quickly saturated popular culture: runner’s high. Long-distance runners described a sudden state of bliss, a disappearance of pain, and a reduction in anxiety that occurred after miles of pushing their physical limits. Intrigued, scientists sought a biological explanation, focusing their attention on newly discovered endorphins.

In 1979, health researcher Gary Egger conducted a daring self-experiment by measuring his pain threshold after running twenty laps of a cricket pitch. For six months, he regularly endured discomfort by tightening a blood pressure cuff on his arm to see how long he could withstand the pain before and after a run. The results were promising: his pain tolerance increased by 50% post-workout. While Egger’s experiment suggested running acted as a natural anesthetic, science soon hit a wall.

The primary issue with the endorphin hypothesis was that these molecules are hydrophilic, meaning they struggle to cross the blood-brain barrier. This implied that high endorphin levels measured in a runner’s blood didn’t necessarily mean they were present in the brain. For three decades, research remained at a standstill, and the runner’s high was largely categorized as a psychological phenomenon that could not be objectively measured in the central nervous system.

PET Scans Change Everything: The First Evidence from Inside the Brain

The real breakthrough came in 2008, when a team of German scientists led by Professor Henning Boecker at the University of Bonn published the results of a pioneering study. Researchers utilized advanced Positron Emission Tomography (PET) technology to settle the endorphin debate once and for all. It was a logistically complex task that required total discipline from the athletes involved.

The study involved ten runners whose brains were scanned before and after a two-hour moderate-intensity run. Scientists used a special radioactive tracer that bound to the same opioid receptors as endorphins. If there were fewer receptors available for the tracer after the run, it meant they had been occupied by the body’s naturally released substances. The results were undeniable: running triggered a massive release of endorphins in the limbic and prefrontal regions of the brain.

More importantly, researchers found a direct correlation between the intensity of this chemical release and the subjective feelings of euphoria reported by the athletes. Boecker’s work was a milestone, providing the first proof that the runner’s high has a neurochemical foundation in the human brain. However, the story took a new turn when other researchers asked: are endorphins the only players?

A New Player: Why Endocannabinoids Took Over the Narrative

While the sports world celebrated proof of the “endorphin rush,” neurobiologists like Arne Dietrich began pointing to a different, potentially more significant factor: endocannabinoids. These are natural compounds produced by our bodies that are structurally similar to THC found in cannabis. Unlike endorphins, endocannabinoids are lipophilic, allowing them to cross into the brain almost instantly.

As early as 2003, studies showed that moderate-intensity running caused a significant spike in anandamide—the primary endocannabinoid—in the blood plasma. This discovery suggested these molecules might be responsible for the relaxing, anti-anxiety, and mood-boosting effects of exercise. Soon, scientists suspected that endorphins might only be one piece of a complex chemical puzzle.

This hypothesis was confirmed by a 2015 study on mice led by Johannes Fuss. Researchers demonstrated that blocking cannabinoid receptors in rodents completely eliminated the effects of euphoria and anxiety reduction after running. Meanwhile, blocking opioid receptors had no effect on these sensations. This suggested that while endorphins are present in a runner’s brain, endocannabinoids are likely the ones pulling the strings regarding emotional well-being.

Euphoria as Adaptation: A Biological Reward for Survival

In 2012, anthropologist David A. Raichlen proposed a bold theory: the runner’s high is an evolutionary reward for physical effort that allowed our ancestors to survive as hunter-gatherers. Raichlen conducted a fascinating comparative study, analyzing endocannabinoid levels after running in humans, dogs (natural long-distance runners), and ferrets (sedentary animals). The results were striking: the runner’s high only occurred in species adapted for running.

The study showed that in humans and dogs, anandamide levels surged after intense exertion, while in ferrets, they remained unchanged. This suggests that nature equipped us with a neurobiological motivational system to encourage traveling long distances in search of food. Without this chemical “reward,” the energy costs and injury risks might have discouraged early humans from hunting.

Modern research, such as the 2021 study by Michael Siebers, finally confirmed these findings in humans. Using naltrexone—a drug that blocks opioid receptors—researchers proved that runners still experienced euphoria and reduced anxiety even when their endorphin system was completely “switched off.” This solidified the role of endocannabinoids as the key element of runner biology.

From Hypotheses to Therapy: How the Runner’s High Changed Science

The history of discovering the runner’s high is a journey from skepticism to endorphin fascination, and finally to an understanding of deep endocannabinoid mechanisms rooted in our evolution. Today, we know that the runner’s high is not a myth, but a precise biological mechanism that improves mood and protects the brain from stress and depression. It is scientific validation for what runners have always known: our bodies are built to move, and our minds are wired to reward us generously for the effort.