Mood disorders are disorders in which the fundamental disturbance is a change in affect or mood, to depression (with or without associated anxiety) or to mania. Major changes in mood are usually accompanied by a change in the overall level of activity. Most of these disorders tend to be recurrent, and the onset of individual episodes can often be related to stressful events or situations. The mood disorders include major depressive disorder (MDD), its milder form dysthymic disorder, bipolar disorder (BD) and its less severe form cyclothymic disorder.