The Science Behind the Runner’s High: What Really Happens in Your Brain

by GRP Trail Runner Hannah Rowe

For many runners, there's a moment when everything clicks: the body relaxes, the mind clears, and running becomes almost effortless. This euphoric state, often called the runner’s high, is a phenomenon as elusive as it is addictive. But what exactly causes it? And why do some people feel it while others chase it for years?

Endorphins: Part of the Puzzle

The popular understanding of the runner’s high has it that endorphins, which are the body’s natural painkillers (and are chemically similar to morphine), are the main cause. And this is partly true. Endorphins are released during prolonged aerobic exercise. These neuropeptides bind to opioid receptors in the brain and are known to dull pain and promote feelings of pleasure. However, endorphins are large molecules that don’t easily cross the blood-brain barrier, which is the membrane that separates circulating blood from the brain’s fluid environment. That raised a question: if endorphins can’t easily get into the brain, are they really behind the runner’s high? Using PET scans, scientists have shown increased opioid receptor activity in the brains of runners, suggesting that endorphins may not be the only player in the game.

The Endocannabinoid System: A New Frontier

Recently, attention has shifted to another class of molecules: endocannabinoids. These are naturally occurring chemicals in the body that resemble the active compounds in cannabis (like THC). Unlike endorphins, endocannabinoids can cross the blood-brain barrier easily and influence mood, memory, and pain perception. A 2015 study in Proceedings of the National Academy of Sciences found elevated levels of anandamide - an endocannabinoid linked to mood regulation - in mice after running. When the researchers blocked endocannabinoid receptors, the mice didn’t show the usual signs of reduced anxiety or pain after exercise. This evidence strengthens the case that the endocannabinoid system plays a central role in producing the runner’s high.

Other Neurobiological Effects of Running

Regular exercise elevates the brain’s levels of dopamine and serotonin, which are two neurotransmitters involved in reward, mood regulation, and motivation. Over the long term, consistent running has been linked to increased hippocampal neurogenesis (the growth of new brain cells), greater resilience to stress, and lower levels of anxiety and depression. Although these neural adaptations do not create the immediate euphoria of a “runner’s high,” they help explain why sustained running steadily improves mental health.

Why Don’t All Runners Feel It?

Not everyone gets this special privilege of experiencing a runner’s high, as several factors influence its onset:

1. Intensity and duration: Endocannabinoid and dopamine surges show up most reliably after moderate-to-high-intensity aerobic exercise sustained for at least 30-45 minutes. Short, very easy, or stop-and-go efforts usually don’t push the metabolic and neurochemical systems far enough; extremely hard efforts, on the other hand, can generate fatigue and stress hormones that blunt the mood boost. Studies find that runs of ~60 minutes at tempo-like effort maximize circulating the compounds now thought to underlie the high. 

2. Genetics: Individual variation in genes shape how strongly people feel exercise-induced euphoria. Some polymorphisms make receptors more (or less) sensitive to the same spike in endorphins or endocannabinoids, so two runners doing the identical workout may experience very different mood responses.

3. Mental state before the run: Starting a workout already stressed, anxious, or emotionally aroused can amplify the contrast once endocannabinoids kick in, so the mood lift feels more dramatic. Conversely, beginning a run in a calm or distracted state may make any incremental change harder to notice. Research on endocannabinoids show they are tightly linked to stress regulation and relief, which explains why the same biochemical surge can feel stronger when a person is trying to “run off” tension.

4. Training status: New or de-trained runners often battle muscle discomfort, inefficient stride, and cardiorespiratory strain long before 30 minutes have passed. So the brain is busy processing pain signals rather than registering euphoria. Seasoned runners, with better economy and pacing, can stay in the metabolic “sweet spot” long enough for endocannabinoids to accumulate and be felt. Surveys consistently show that first-time runners report the high far less often than habitual endurance athletes.

Taken together, these factors explain why a runner’s high is hit-or-miss: it depends on reaching an intensity/duration threshold, having neurochemistry wired to respond strongly, running in the right head-space, and having enough fitness to get through the uncomfortable early miles.

The Takeaway

The runner’s high isn’t a myth - it’s a real, measurable shift in brain chemistry. While endorphins may contribute, endocannabinoids likely play the starring role, acting as natural mood lifters and painkillers. Whether or not you’ve felt it, consistent running can profoundly shape your brain for the better. And I wish that you all will, one day, hit that magical state where your legs move effortlessly and your mind soars!