Depression is a medical disorder characterized by feelings of sadness, hopelessness, pessimism, guilt, and a general loss of interest in life, combined with a sense of reduced emotional well-being and low energy. People with depression also tend to have disturbed sleep, reduced appetite and may even feel suicidal.

Symptoms of depression can occur as a result of a wide range of medical and psychiatric disorders. In a person who has been found to not be medically ill, there are three common...