Exercise benefits not only your body, but your mind as well. Aerobic exercise prompts the release of mood-lifting hormones, which relieve stress and promote a sense of well-being. The repetitive cycle of muscle contraction and release that takes place in yoga or in aerobic pursuits such as walking and swimming increases the level of the brain chemical serotonin, which combats negative feelings. Indeed, there's reason to believe that physical activity might have a positive impact on a number of mental illnesses, including sleep and eating disorders, dementia, substance abuse, and personality disorders. But the most thoroughly studied in this regard are depression and anxiety disorder.

Each year, more than 1 in 10 American adults—19 million total—experience some form of depression. Women are twice as likely to have such depressive episodes. Suicide, the most serious consequence of depression, is the ninth-leading cause of death in this country, and mental illnesses cost the healthcare system $148 billion dollars each year.

Can boost mood

The strongest evidence for the mood-lifting potential of exercise derives from people with symptoms of depression and anxiety. But studies that involved people without such symptoms—including research over the past 15 years that focused on cross-sections of the population in the United States, Canada, Finland, and Germany—have reached roughly similar conclusions. In sum, people getting at least a moderate amount of exercise have fewer symptoms of depression.

Another series of studies has examined the role of regular activity in preventing depression. One such investigation, involving 2,084 men and women age 65 or older, found that among participants who initially reported symptoms of depression, after three years, those who walked daily had a third fewer symptoms. Men tracked in the Harvard Alumni Health Study showed similar results. Those who burned 1,000–2,500 calories per week in aerobic activity lowered their risk for depression by 17%. When participants' weekly energy expenditure topped 2,500 calories, their chances of experiencing depression dropped by 28%.

Exercise vs. medication

Some studies have found that exercise can be as effective as medication in relieving symptoms of mild to moderate depression. But the optimal type, frequency, and intensity of such exercise have yet to be pinned down. Results of one study showed that participants' moods benefited more from three days a week of exercise than from one exercise session per week. This study examined the benefits of aerobic exercise, but others have found that resistance training may also be effective in reducing symptoms of depression.

Aerobic activity can ease anxiety

Exercise can also alleviate anxiety, which is characterized by worry and fear. The greatest benefits derive from a program of aerobic activity lasting at least 10 weeks, and, ideally, more than 15 weeks. People who start with high levels of anxiety and low levels of fitness garner the greatest gains. Research suggests that this type of program can be as successful as other anxiety-reducing techniques, such relaxation training.