So, What is Runner's High?
After extensive study of the phenomenon, Temple University Sport Psychologist Dr. Michael Sachs concluded that runners high is a euphoric state experienced during running, usually unexpected, in which the runner feels an increased sense of well being, an enhanced appreciation of nature, and a transcendence of time and space. But individually reported experiences of such a "state" vary considerably among athletes, ranging from being merely pleasant, to intense joy and euphoria, to experiencing spirituality, to a sense of power and invincibility, to feelings that are similar to the out-of-body sensations induced by drugs, to sexual orgasm. In other words, it covers just about all the feelings that fit the general idea of "feeling good". Not a clear-set definition. And then, there is also the debate about the causes of runner's high.

Is it Physiological or Psychological?
On the physiological side, the suspects are endorphin and serotonin, both of which are glandular chemicals. Researchers have been investigating their relationships with the euphoric feelings associated with exercise.
After being released by the pituitary gland, endorphin attaches itself to specific receptor sites in the brain and affects the brain's perception of pain. For this reason, endorphin is also known as the "natural painkiller". In addition, endorphin also appears to affect mood, memory retention, and learning. Release of endorphin is increased when the body is under stress and pain, e.g., during prolonged exercise. It is unclear, however, if the elevated level of endorphin release is responsible for the positive mood change experienced during an exercise high. First of all, it is too difficult to measure the amount of endorphin released into the brain to distinguish its effect from the effect of other factors. Secondly, the relationship between endorphin and happy feeling is not clear. If endorphin is indeed responsible for mood changes, does it always induce a positive mood or simply a mood change, which can be negative? It is estimated that only about 10% of the people who exercise ever experience a high. There are people who do not experience exercise-related mood changes. Still, there are others who actually report negative emotions during exercise.
Serotonin is another neurotransmitter that helps get messages across nerve cells in the brain. It is found to be affecting a wide range of conditions, including suppressed appetite, migraine headaches, depression, aggression, sleep disturbances, mood shifts, and anxiety. Some drugs, such as Prozac, achieve their mood-altering effect by increasing or suppressing the activity of serotonin in the brain. Now researchers are suggesting that exercise works in a similar way. But since research in this area has just begun, no definite conclusions have been made.

On the psychological side, some investigators point out that what make athletes and exercisers feel good are not some brain chemicals, but are an increased sense of self-confidence and an improved self-image. Many people reported that by being physically active they feel stronger, slimmer, firmer, more in control, and more together. Vise versa, a physically active self-image can lead to dramatic changes in people's exercise habits. For example, Dr. Edward McAuley and his colleagues in the Department of Kinesiology at the University of Illinois in Urbana, Illinois, have conducted an experiment, where they made a randomly-selected group of participants feel good about riding stationary bikes by simply telling them that they have a fitness level that is higher than average. Other researchers suggest that exercise makes people feel good because it provides people with a distraction or break from the hustle and bustle of their everyday life.
Studies like the above certainly suggest that psychological factors are at least one source of the pleasant feelings people experience during and after exercise. They do not seem, however, to be able to rule out the possibility of involvement of other factors.

What Do I Think?
Based on other peoples' testimony and my own experience, I am inclined to say that runner's high, as a unique experience, does exist. Sure, there is no clear definition for it, but like with many natural phenomena in this world, you know it when you feel it. After all, how many people have ever found words to be adequate in describing a high -- any high, whether it is induced by spirituality, drug, or sex?
It seems to me, also, that the cause of runner's high is a combination of several factors. This is not surprising, considering not only that all sports activities require physical, mental, and emotional involvement, but also that human emotion itself is both psychological and physiological in nature. Studies of the causes of exercise high can help us understand the complex relationship among various factors. For example, how does our mental state affect our physical capabilities? And how does physical exercise contribute to our emotional well-being? Clinical practitioners and counseling professionals will also find significant value in these studies, when they apply the findings to treating patients with exercise addiction or athletes in need of higher motivation. In fact, if physiologists and psychologists can find a way for more people to achieve exercise high, they may just make the greatest contribution to a society that will, as a result, become not only fitter but also happier.
Bron: Psychology.com