Dysthymic Disorder
This is a mood disorder is a less severe form of depression. Although less extreme, dysthymic disorder causes chronic of long lasting moodiness. Low, dark moods invade your life nearly every day for two years of more. Dysthymia is contrasted with a major depressive episode that last two years or longer, which is called chronic major depression. This disorder can occur alone or along with other psychiatric or mood disorders. This disorder is more common in women than in men. Family history of mood disorders is not uncommon. It can also appear earlier than major depression, and it can begin anytime from childhood to later in life. Around 5% of the general population is affected by this disorder.
The causes are not well understood. Factors that conspires to create this mood disorder may include: * Genetics * Abnormalities in the functioning of brain circuits involve in emotional processing * Chronic stress or medical illness * Isolation * Problems adjusting to life stresses

Symptoms: * Feelings of hopelessness or helplessness * Trouble sleeping or daytime sleeping * Eating too much or not enough * Low energy * Low self esteem * Trouble concentrating, making decisions
You may also tend to withdraw more and have stronger feelings of pessimism and inadequacy than with major depression.

Treatments: * Psychotherapy (talk therapy) is generally considered the treatment of choice for dysthymic disorder. * Treatment approach may include antidepressant medication in addition to psychotherapy * Antidepressants such as selective serotonin reuptake inhibitors * Trycyclic antidepressants * Aerobic exercise can help with mood disorders, this is most affective when done four to six times a week

References:
http://www.webmd.com/mental-health/mood-disorders