NIH - Medical Encyclopedia Depression

"Depression may be described as feeling sad, blue, unhappy, miserable, or down in the dumps. Most of us feel this way at one time or another for short periods. But true clinical depression is a mood disorder in which feelings of sadness, loss, anger, or frustration interfere with everyday life for an extended time. … The main types of depression include: Major depression -- five or more symptoms listed above must be present for at least 2 weeks, but major depression tends to continue for at least 6 months. (Depression is classified as minor depression if less than five depressive symptoms are present for at least 2 weeks.) and Dysthymia -- a chronic, generally milder form of depression but lasts longer -- usually as long as two years. Depression also includes atypical depression, postpartum depression, Premenstrual dysphoric disorder (PMDD), and Seasonal affective disorder (SAD). … If you are depressed for 2 weeks or longer, you should contact your doctor, who can offer treatment options. … Call 911, a suicide hotline, or get safely to a nearby emergency room if you have thoughts of suicide, a suicidal plan, or thoughts of harming yourself or others."

Regular exercise, anxiety, depression and personality: A population-based study. (Prev Med. 2006)

"This study corroborates and extends previous findings: regular exercise is cross-sectionally associated with lower neuroticism, anxiety and depression and higher extraversion and sensation seeking in the population."