“Exercise induces changes in mental status, particularly analgesia, sedation, anxiolysis, and a sense of wellbeing. The mechanisms underlying these changes remain unknown.

Recent findings show that exercise increases serum concentrations of endocannabinoids, suggesting a possible explanation for a number of these changes…

At first glance, it appears that the runner’s high phenomenon is, at present, not a scientific problem because it is built on circumstantial evidence and lacks a plausible mechanistic explanation. However, recent data in our laboratory showed that endurance exercise activates the endocannabinoid system, suggesting a new mechanism underlying exercise induced alterations of mental status.”

http://bjsm.bmj.com/content/38/5/536.long