A Heidelberg University team in Mannheim, with Mainz, tested that assumption in mice by blocking either opioid receptors, which endorphins act on, or cannabinoid receptors, and only one of the two blockers erased the calm, pain-dampened state that follows a run.
Johannes Fuss's team found that running raised blood levels of both an opioid and an endocannabinoid, anandamide, a molecule chemically similar to the active compound in cannabis. Blocking cannabinoid receptors eliminated the anxiety relief and pain reduction mice showed after running, while blocking opioid receptors left both effects fully intact.
The result flipped the standard explanation: the calming, pain-blunting part of a runner's high tracked with the body's cannabinoid system, not its endorphin system. Sedation after running was not affected by blocking either receptor, and the subjective sense of euphoria people report cannot be measured directly in mice, so the human experience of a runner's high is still described only partly by this mechanism.
Read the original study
The calm after running is not endorphins.
Source
- Fuss, J., Steinle, J., Bindila, L., Auer, M. K., Kirchherr, H., Lutz, B., & Gass, P. (2015). A runner’s high depends on cannabinoid receptors in mice. Proceedings of the National Academy of Sciences, 112(42), 13105–13108. https://doi.org/10.1073/pnas.1514996112