For decades, scientists have touted the positive relationship between exercise and mood. Exercise appears to influence mood in a number of ways, primarily through the release of chemicals, called neurotransmitters, that relieve pain and lift mood. Physical activity produces a positive effect on the levels of the four key brain chemicals that affect mood: epinephrine, serotonin, dopamine, and endorphins. In addition, 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 the repair of damaged neurons. Since studies show that the hippocampus in depressed individuals can be 15 percent smaller than normal, accelerated neurogenesis through exercise-triggered BDNF may help treat clinical depression.
Epinephrine is the chemical messenger responsible for the body's "fight or flight" response to danger or stress. This chemical is depleted under conditions of chronic stress, anxiety, and starvation, often leading to exhaustion, mental fatigue, and depression. Although intense exercise stimulates the release of epinephrine, regular, moderate exercise with alternating levels of light and intense exertion reduces circulating levels of epinephrine. This facilitates better function of the parasympathetic nervous system, which mediates digestion, sleep, the immune response, and the repair of body tissues. It also lowers heart rate, blood pressure, body temperature, and cortisol levels, all of which have a psychologically calming and energizing effect, which explains part of the link between exercise and mood.
Serotonin is the main neurotransmitter responsible for feelings of well-being and satisfaction. Studies have shown that depressed people have reduced serotonin levels in the brain, and many antidepressant medications work by raising serotonin levels. Low serotonin levels cause irritability, fatigue, and moodiness. In addition to stimulating neurogenesis, BDNF increases serotonin production and signaling, which in turn increases BDNF release. This synergistic cycle may be a critical component of the significant association between exercise and improved mood.
Dopamine helps the body regulate waking and sleeping cycles. An imbalance in dopamine disrupts sleep patterns, leading to fatigue. Jet lag is a common result of dopamine imbalance associated with traveling across time zones, poor diet or starvation, stress or anxiety. In the brain, dopamine levels fluctuate in sync with serotonin levels. Since exercise raises serotonin levels, dopamine levels also rise, especially in response to moderate-intensity, long-duration exercise, further illustrating the connection between exercise and mood.
Endorphins, the body's natural opiate pain relievers, 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, reducing pain and creating a natural effect. Research shows that endorphins spill into the brain within 30 minutes of starting 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 the gratification needed to achieve a high lessens the addictive qualities of exercise. According to various surveys, as few as 15 percent of Americans regularly engage in moderate physical activity and experience the euphoric effects of exercise and the mood-enhancing effects of endorphins.
Although any type of exercise produces an improvement in mood, the type of exercise a person participates in makes a significant difference in the effect it has on brain chemicals. Highly intense activities, such as running, weightlifting, and interval training, increase epinephrine levels. On the other hand, cycling, long-distance running, swimming, and other activities that require moderate exertion for longer periods produce a greater dividend in mood-enhancing brain chemicals. Repeatedly used pathways in the brain are strengthened each time they are used, making the neurochemical response to exercise and the corresponding elevation of mood more profound over time.