Runner's High

Is there a runner's high triggered by environmental stimuli and biological aspects? For me the answer is yes—no question. One runner's observation:

"I've been running for 25 years or so, and don't know for sure what runner's high is. On almost every run, and certainly the long ones, there are periods of contentment or reflection when one is on automatic pilot and the terrain goes past unnoticed. Runner's high? I don't know."