While mood fluctuations in life are normal for all of us, if mood disturbances persist and start to affect our functioning in non-productive ways, we may meet the the criteria for the diagnosis of a mood disorder.
There are two kinds of mood disorders: Depressive Disorders that are also called Unipolar Mood Disorders and Bipolar Mood Disorders.
Depression, in all of its variations, represents one set of conditions in which a disturbance of mood is a primary feature. However, just a sad or low mood without other indicators may not indicate the presence of a depressive disorder. Bipolar disorder, in all of its variations, represents an entirely different set of conditions in which a disturbance of mood is a primary feature. Again, just ups and downs in mood without other indicators may not indicate the presence of a bipolar disorder.
To learn more, watch this video clip recorded on December 16, 2013 for the TV show FOCUS ON in Princeton, NJ.