"copy/paste"

Exercise seems to have an effect on your emotional reaction to stress. It does this by altering your mood. Fit people are usually in high spirits after a lengthy exercise, sometimes to the point of elation or joy. This feeling is associated with the presence of endorphins, which are released by the pituitary gland in the brain.

The word "endorphin" is a combination of "endo" and "morphine" -- meaning endogenously produced morphine, or internally produced painkiller. Endorphins are the body's natural pain reliever. It may be that the brain interprets exercise as a form of "pain" or it may be that the rise in fatty acids caused by long, gentle exercise acidifies the blood, which triggers the release of endorphins.

In any case, you can get from exercise a natural high, similar to a drug high but with none of the bad side effects. People who do long, continuous, gentle exercise enjoy the most effective stress therapy known to man.