Simply put, a mood disorder is a frequent or repeated disturbance of your normal mood. Difficult situations in life bring stress, frustration and disappointment. These are but normal reactions to the hardship that you may face in life.
However, if these feelings last more than a few weeks or months wherein it spoils your quality of life, it could be a sign of a mood disorder. You may feel sad all the time. Or you may find it difficult to fall asleep or stay asleep. You may experience various symptoms of illness that may not really have an identifiable physical source.
