For decades, scientists have touted the positive relationship between exercise and mood. Exercise appears to influence mood in several ways, mostly by the release of chemicals, called neurotransmitters, that ease pain and lift spirits. Physical activity produces a positive effect on the levels of the four key brain chemicals that impact mood: epinephrine, serotonin, dopamine, and the endorphins. Furthermore, exercise stimulates the release of brain-derived neurotrophic factor (BDNF) in the hippocampus region of the brain, which stimulates the generation of new neurons and repair of damaged neurons. Since studies show that the hippocampus in depressed individuals may be 15 percent smaller than normal, accelerated neurogenesis through exercise-triggered BDNF may help to treat clinical depression.
Epinephrine is the chemical messenger responsible for the body's "fight or flight" response to danger or stress. This chemical becomes depleted under conditions of chronic stress, anxiety, and starvation, often producing exhaustion, mental fatigue, and depression. Although intense exercise stimulates release of epinephrine, regular, moderate exercise with alternating light and intense levels of exertion lowers circulating epinephrine levels. This facilitates better function of the parasympathetic nervous system, which mediates digestion, sleep, immune response, and repair of body tissues. It also decreases the heart rate, blood pressure, body temperature, and cortisol levels, all of which has a psychologically calming and energizing effect, explaining part of the link between exercise and mood.
Serotonin is the principal neurotransmitter responsible for feelings of well being and satisfaction. Studies have shown that depressed persons have reduced brain serotonin levels, and many of the antidepressant drugs work by elevating serotonin levels. Low serotonin levels lead to irritability, fatigue, and moodiness. In addition to stimulating neurogenesis, BDNF boosts serotonin production and signaling, which, in turn, heightens BDNF release. This synergistic cycle may be a critical component of the significant association between exercise and mood enhancement.
Dopamine helps the body to regulate waking and sleeping cycles. An imbalance in dopamine disrupts sound sleep patterns, producing fatigue. Jet lag is a common result of dopamine imbalance associated with travel across time zones, poor diet or starvation, stress, or anxiety. In the brain, dopamine levels fluctuate in sync with serotonin levels. Since exercise elevates serotonin levels, dopamine levels also increase, especially in response to moderate intensity, long-duration exercise, further illustrating the connection between exercise and mood.
Endorphins, the body's natural opiate painkillers, reduce physical and mental pain and produce a form of euphoria. These chemicals, which are released by the pituitary gland in response to pain or stress associated with exercise, interact with the same receptors as heroin or morphine, cutting pain and generating a natural high. Research shows that endorphins spill into the brain within 30 minutes of the initiation of physical activity, and this influx increases with the frequency of exercise. Although endorphins can have an addictive effect, much like nicotine or morphine, the 30-minute delay in gratification necessary in order to achieve a high lessens the addictive qualities of exercise. According to several surveys, as few as 15 percent of Americans regularly engage in moderate physical activity and experience the euphoric effects of exercise and mood enhancement by endorphins.
Although any kind of exercise produces mood enhancement, the type of exercise in which a person engages makes a significant difference in the effect it has on brain chemicals. Highly intense activities, such as sprinting, weight lifting, and interval training, increase epinephrine levels. On the other hand, cycling, long-distance running, swimming, and other activities requiring moderate effort over longer periods of time produce a greater dividend in mood-enhancing brain chemicals. Pathways that are repetitively used in the brain become stronger each time they are used, making the neurochemical response to exercise and corresponding mood elevation more profound over time.