It's known that a runner's high isn't actually the result of endorphins, which are too large to cross the blood-brain barrier, but scientists have long sought the elusive cause of the much-talked-about sense of euphoria and calm that can accompany the end of a workout. Now researchers are reporting in the Proceedings of the National Academy of Sciences that endocannabinoids—chemicals that the New York Times describes as, "essentially, internally produced marijuana"—are behind the subtly intoxicating effects of exercise. To test this, researchers conditioned mice to run on a treadmill, then let one group run for five hours and forced the second to be inactive. All mice were then watched for signs of anxiety (i.e. preference for darkness, pain intolerance, etc.).
Then, in a second experiment that looked nearly identical, the researchers added endocannabinoid and endorphin antagonists to block their effects. This time the mice who ran did not exhibit the reduced anxiety associated with a runner's high, reports CBS Atlanta. The results can't be directly translated to humans, but the fact that such tiny animals have to run three miles a day to produce a runner's high may suggest that humans have evolved to be in motion. "The subtle upshot of the new study may be that we should run," writes Gretchen Reynolds in the Times. "And if we don’t feel a high, perhaps try running more, until eventually a gentle euphoria may settle in and we can turn to our running companion and say, 'Ah, my endocannabinoids are kicking in at last!'" (Endocannabinoids are implicated in over-eating, too.)