What is "Runner's High?":

"Runner’s high has been described subjectively as pure happiness, elation, a feeling of unity with one’s self and/or nature, endless peacefulness, inner harmony, boundless energy, and a reduction in pain sensation."

British Journal of Sports Medicine - Review: Endocannabinoids and Excercise  2004;38:536-541  doi:10.1136/bjsm.2004.011718

The "Endocannabinoid" System?:

"As the name suggests, endocannabinoids are chemicals that, like cannabis in marijuana, alter and lighten moods. But the body produces endocannabinoids naturally. In other studies, endocannabinoid levels have been shown to increase after prolonged running and cycling, leading many scientists to conclude that endocannabinoids help to create runner’s high."

The New York Times, Herald-Tribune 04/12: "The Evolution of the Runner’s High"

How Endocannabinoids Affect Volentary Excerise

("an ancient human trait?")

"It is possible that the more sedentary groups cannot adequately exercise at the intensities required to elecit a siginificant eCB elevation, and do not gain similar psychological benefits from exercise at lower intensities. This possible intra-specific variation in physical fitness may explain why some individuals do not enjoy exericse."

"It is possible that neuobiological rewards are induced by eCB signaling and are an ancient human trait that evolved to encourage aerobic activity," reports The Journal of Experimental Biology."

Raichlen et al. Wired to run: exercise-induced endocannabinoid signaling in humans and cursorial mammals with implications for the ‘runner’s high’. J Exp Biol. 2012 Apr 15;215(Pt 8):1331-6.

" Humans report a wide range of neurobiological rewards following moderate and intense aerobic activity, popularly referred to as the 'runner's high', which may function to encourage habitual aerobic exercise. Endocannabinoids (eCBs) are endogenous neurotransmitters that appear to play a major role in generating these rewards by activating cannabinoid receptors in brain reward regions during and after exercise.

Raichlen et al. Wired to run: exercise-induced endocannabinoid signaling in humans and cursorial mammals with implications for the ‘runner’s high’. Journal of Experimental Biology. 2012 Apr 15;215(Pt 8):1331-6.