There's possibly no better feeling than the calm and happiness that follow thecompletion of a long, tough run.So where does this this so-called "runner's high" come from'You've probably heard it getchalked up to a rise in endorphins, the "happy" chemicals that induce feelings of pain relief and pleasure. But it's actually more complicated than that.The 'endorphins make you happy'ideaThe idea that increased levels ofendorphins are responsiblefor that post-workouthappy feelingcame out of 1980s research that showed endorphin levels in the blood spiked after prolonged exercise. Some researchers assumedthese chemicals must also producethe sense of euphoria we feel after a workout.But recent studies in mice suggest that endorphins actually mightnot haveanything to do with the runner's high. Theproblem with the endorphinexplanation is that they're very largemoleculesso large, in fact, that they can'tmove from the bloodinto the brain.The blood-brain barrier is key to keeping the brain safe, since it stopscertain pathogensand molecules from passing from the blood into the brain. Because endorphins can't get through, it's unlikely that theyare the solechemicalresponsible for the feelings associated with vigorous exercise.Instead, scientists think the effect canbe attributed to other chemicals in the body that produce similar pain-relieving and happy feelings.Turning to endocannabinoidsLevels of a chemical called anandamide also increase when you exercise, according to a2015 study in miceand a small 2004 studyin people.Anandamide is a type of endocannabinoid, a chemical that's part of thesystem that moderates the psychoactive, feel-good effects of marijuana. And unlike cumbersome endorphins, anandamide can smoothly make its way from the blood to the brain.Forthe 2015 paper, researchers at the Central Institute of Mental Health at the University of Heidelberg medical school compared the effects ofendorphins and endocannabinoidsonmice as they ran on running wheels.The researchers found that, in addition toappearing more calm lesssensitive to pain after running, the micehad higher levels of both endorphins and endocannabinoids. Theanimals also spent more time in well-lit parts of their cage, something calm, less anxious mice tend to do. They were also slightly more pain-tolerant after their stints on the wheel.To measure the effects of each chemical individually, the researchers gave the mice drugs thatblocked the effects of each. When they blocked the endorphins, nothing happened the animalsremained more relaxed and pain-tolerant. But when they blocked the effects of the endocannabinoids, the symptoms of the mice'srunner's highs disappeared.These findings suggestthat the mice's elevated endorphinlevels had littleto do with their post-workout buzz.This research has one obvious caveat, however: Mice aren't humans. And the study also revealed, disappointingly, that you probably need to run pretty far to experience a runner's high. The mice ran an average of more than three human miles per day (a long wayfor a mouse).If you're interested in learning more about the connection between endocannabinoids and runners' highs, this video from YouTube channel SciShowwalks through someof the evidence.Otherfactors at playOther studies, however, suggest that neither endorphins nor endocannibinoids are the cause of the runner's high. A 2015study, for example,found that mice withlow levels of a hormone called leptin tendedto run farther than mice with normal levels of leptin.Leptin, otherwise known as the "satiety hormone," inhibits thefeeling of hunger in order to regulate our energy levels. The idea is that the less full (or more hungry) you feel, the more motivated you are to keep running. And that increased motivation might make it easier to get a runner's high."Ultimately, leptin is sending the brain a clear message: When food is scarce, its fun to run to chase some down," lead study author Maria Fernanda Fernandes told Outside Magazinein 2015.But again, the fact that these results have been demonstratedin mice doesn't mean the same effects willnecessarily be found inhumans. And because theremight be a combination of factors at play, definitive evidenceof whatexactly causes arunner's high might continue to elude scientists for a while.An earlier version of this post was written by Tanya Lewis.NEXT UP:8 killer arm workouts you can do virtually anywhereSEE ALSO:13 exercises to keep your legs strong at any ageJoin the conversation about this storyNOW WATCH: Scientists say this may be the best exercise for your brain health
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