Mood disorders, sometimes called affective disorders, involve severe and persistent disturbances of mood that endure despite the influence of external events. Everybody's mood varies according to events in the world around them. In mood disorders our normal emotions are polarised into more severe forms. Happiness can become mania and sadness can become profound despair or depression. This polarisation can be called a bipole, with mania at one extreme and depression at the other. When patients suffer manic episodes and depressive episodes they are said to have bipolar disorder. The American classification system (DSM) echoes Kraepelin's original idea that the psychotic disorders broke down into either schizophrenia or manic depression. Based on reviews of available research, the DSM system recognises a bipolar (I) disorder and a bipolar (II) disorder. Bipolar (I) disorder involves recurrent episodes of depression and bipolar (II) disorder involves recurrent episodes of mania and depression.