(NEWSER) – 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.).