For decades, endorphins have hogged the credit for producing "runner's high," that fleeting sense of euphoria and calm that many people report experiencing after prolonged exercise. Who among us, after an especially satisfying workout, hasn't thought, "ah, my endorphins are kicking in." Endorphins are the world’s sole celebrity peptide. ... Finding endorphins in the bloodstream after exercise could not, however, constitute proof that the substance was having an effect on the mind. So researchers started to look for other candidates to help explain runner’s high.