They call it “runner’s high.”

It’s that feeling you get, post-workout, when your bad mood has shifted to good or your good mood has gotten even better.  Dedicated gym-goers claim to be addicted to it.

We’ve long credited endorphins for the high that follows a hard workout, but that may be a wrong assumption, according to the New York Times.  Endorphins, in fact, are too large to pass through the blood-brain barrier and therefore likely have little effect on the mind.

So it’s time to give credit where credit is due.  A series of studies seem to suggest it’s actually the endocannabinoid system that acts as a mood enhancer.

The first study that linked endocannabinoids with runner’s high was authored by scientists at the Georgia Institute of Technology in 2003.  They found that 50 minutes of running on a treadmill increased blood levels of endocannabinoid molecules in college students.

Unlike endorphins, cannabinoids are small enough to cross the blood-brain barrier, and therefore could actively affect the brain.

Follow-up studies found that endocannabinoid systems lit up in rats when they ran on a treadmill and drank sugar water – two typically pleasurable activities – and that rats with impaired endocannabinoid systems ran for only half the time that rats with functioning systems did.

However, as Francis Chaouloff, a researcher at the University of Bordeaux in France and lead author of the genetically modified mouse study, points out: “Rats do not fill questionnaires to express their feelings related to running,” and runner’s high is a subjective human experience.

Click here to read more from the New York Times.