The black silhouette of a person running outdoors at dusk.

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The infamous “runner’s high” athletes experience post-exercise has long been thought to be the product of endorphins, those little hormones that lift your mood and make you happy. But now, new research suggests that the runner’s high doesn’t come from endorphins: rather, it may come from the body’s own endocannabinoids, chemicals akin to those in marijuana.

German researches have shown that the endocannabinoid system at the very least plays a part in the runner’s high. They found that endorphins can’t pass through the blood-brain barrier, but a lipid-soluble endocannabinoid, anandamide, can travel from the blood into the brain, where it has the ability to give you that happy-floaty feeling.

To conduct the study, researches with the Central Institute of Mental Health at the University of Heidelberg monitored mice, as so many experiments often do. The researchers tested the mice’s anxiety levels by putting them in cages with pockets of both darkness and light and then gave them running wheels. The mice were found to be more tranquil after running and they were less anxious.

When the researchers used drugs to block parts of the mice’s endocannabinoid systems, they were decidedly less chill after using the running wheel. This would not have happened if the runner’s high were the result just of endorphins; this suggests that the endocannabinoids are responsible for that happy feeling instead.

But there are downsides to the discovery, too. “We may need to cover considerable mileage in order to experience a runner’s high; the mice in the study, small as they are, averaged more than three miles every day on their wheels,” says the New York Times. We should probably be running and getting any exercise at all, but many runners still swear that the high is pretty easily accessible.Some other things that may contribute to the runner’s high, like euphoria, were deemed to be too subjective to study with the mice for this experiment.

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Mary Summers is a recent college grad and freelance writer residing in the Pacific Northwest. She loves writing about trending topics, health and beauty advice, music, film, and television.