People who exercise regularly know that they feel better after a workout. After a day full of aggravation and annoyance, a bout of running or swimming can be a real attitude adjuster. While an intensely competitive hour of basketball or cycling might be one person's ideal exercise, a quiet walk with a dog might be right for someone else. The important element is physical activity. Aerobic exercise performed at an appropriate intensity will reward people with a general feeling of well-being.

In 2009 the British Medical Journal published an analysis of the relationship between fitness levels and mental health. The researchers examined years' worth of epidemiological data from the Scottish Health Service, analyzing the health and activity histories of more than 19,000 men and women. Out of this randomly selected group, about 3,200 people had documented psychological distress. The researchers found a statistically significant, positive correlation between engaging in regular physical activity and good mental health. People who exercised regularly had better psychological profiles than those who did not. Moreover, they were able to show a "dose-response" relationship. Those who exercised more frequently, and at a higher intensity, were even less likely to experience psychological distress than those who exercised at low or moderate intensity for shorter periods of time. Of course, the appropriate level of activity for any one person must be decided on an individual basis, in consultation with a doctor.

Scientists are figuring out why this connection exists at the molecular level. One well-documented link is a neurotransmitter called serotonin. Higher levels of serotonin correlate to improved mood, whereas lower levels of this neurotransmitter correlate to anxiety, depression and anger. Drugs that increase the amount of serotonin in the brain (selective serotonin re-uptake inhibitors, or SSRIs) are commonly prescribed as anti-depressant medications. But research has shown that aerobic exercise also increases the level of serotonin in the brain, highlighting at least one of the pathways between exercise and the feeling of wellness that comes with it. In fact, a research group at Duke University Medical Center has been studying this connection for over a decade, and their results indicate that, for many people, exercise can be at least as effective as standard drug treatment in the struggle against depression.

Another link between exercise and mood is the hormone cortisol, which the body produces naturally as a response to stress. Back in our cave-man days stress usually resulted in fighting enemies or running as fast as we could to survive. Now stress is usually linked to something sedentary, like a traffic jam or an argument with the boss. In people who feel chronically under stress, regardless of its source, their levels of cortisol remain high. Exercise is known to reduce the level of cortisol, which in turn reduces the feeling of "being stressed out" and brings about a calming effect.

A recent study from the University of Georgia looked at a group of young men with "high trait anger"—that is, men who got very angry with little provocation. They were shown pictures designed to incite feelings of anger and their responses were scored using standard techniques. Then half of the group underwent a single, 30-minute session of moderate exercise, while the other half was allowed to sit quietly for those 30 minutes. Upon a second viewing of the same images, the sedentary group's measured anger was much more severe, but exercise group's anger profiles remained level. While exercise did not make them any less angry to see these disturbing images, it seemed to help the men keep their anger under control.

While some scientists are studying the body's response to exercise on a molecular level, others offer more holistic reasons that increased physical activity could improve mental health. For example, participation in sports brings with it a lot of interpersonal connections between teammates or others with similar interests. Also, people who exercise tend to be in better physical health. Poor physical condition often causes or exacerbates feelings of anxiety or depression, and so better physical health can often elicit improvements in psychological health as well. Finally, exercise usually brings about positive changes in one's body, in terms of weight loss, muscle gain, improved posture, etc. These changes, in turn, make people feel more capable, raise self esteem and improve social interactions. All of these factors can positively affect psychological health too.

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