The endorphins and cannabinoids we produce in our brains, which contribute to pain reduction and to feelings of euphoria, play a role in the effect. A 2008 study found that two hours of running increased endorphin levels in athletes’ brains. And higher levels of endorphins were associated with stronger feelings of euphoria. Chaouloff and his colleagues have shown that removing cannabinoid receptors in the brains of mice decreases their running. In humans and dogs, Raichlan’s group has shown that 30 minutes of running boosts blood levels of the brain’s natural cannabinoid, anandamide.