The "runner's high" is an actual thing, and until recently it was commonly associated with the release of endorphins, hormones that essentially function as "pleasant painkillers." But scientists from the Central Institute of Mental Health at the University of Heidelberg medical school in Germany have linked the runner's high to the endocannabinoid system.
Researchers had some lab mice run on a wheel (hopefully outfitting them in tiny adorable running shoes and shorts) and noticed elevated levels of both endorphins and endocannabinoids after their run. The mice acted less anxious and were better able to tolerate pain. Blocking endorphin receptors didn't change their behavior, and the mice still experienced the runner's high.
However, when the scientists blocked the mice's endocannabinoid systems, the little Pre-furr-taines were just as anxious after their runs as they were before running, and they were also more sensitive to pain. Additionally, the feeling of the runner's high only kicked in after a certain amount of mileage. Mice who ran more than three miles a day exhibited less anxious behavior. (I'm not sure what that equates to in human miles, but as someone who's run a number of marathons and half marathons, I can tell you it definitely kicks in during longer-distance races.)
Theoretical physicist and cosmologist Marcelo Gleiser opines that this study makes sense from an evolutionary standpoint:
"We evolved to hunt fast moving animals and to escape from equally fast predators. Such prolonged runs are tiring and painful. If we get a reward from working hard, our chances of running longer and faster are increased. And, with them, our chances of survival. As Christopher McDougall pointed out in Born to Run, his inspiring book about long-distance running, running is in our DNA, in how our bodies evolved to have an Achilles tendon and sweat glands and a series of other adaptations that make us all into potential marathoners."
That's right, your body has actually evolved into an efficient running machine. And not only does running mimic the high you get from your favorite strain, it's healthy for you. Talk about a win-win!