Several studies have been demonstrating that regular physical exercise can improve mood in people with mild and moderate depression, as well have longer lasting effects and could be an acceptable substitute for antidepressants as per a study published in the Archives of Internal Medicine. A follow up to this study found that the people who exercised regularly after completing the study, regardless of which treatment they were on originally, were less likely to relapse into depression. Another study published in 2005 found that fast walking about 35 minutes daily five days a week or 60 minutes three times a week significantly improved mild to moderate depression symptoms. Physical exercise stimulates the production and action of the endorphins, chemicals that improve natural immunity and pain perception. The endorphins also improve mood as well as another chemical known as norepinephrine. Regular exercise also lowers the blood pressure, protects against heart disease and cancer, and boosts self-esteem.