One theory for some of the benefits of exercise include the fact that exercise triggers the production of endorphins. These natural opiates are chemically similar to morphine. They may be produced as natural pain relievers in response to the shock that the body receives during exercise. However, researchers are beginning to question whether endorphins improve mood. Studies are showing that the body's metabolism of endorphins is complex, and there are likely additional mechanisms involved in the mental health effects of exercise.