Depression is defined by a feeling of sadness or loss of interest in daily life which lasts all day, every day for at least two weeks. It is accompanied by changes in sleep and appetite, feelings of worthlessness, hopelessness, loss of energy, and difficulty concentrating or making decisions. In more severe cases, one may think about dying or suicide. In less severe depression, a person may have some but not all of these symptoms, or may have a general sense of sadness without losing the ability to function.
CBT for depression works on increasing behaviors which improve mood, and on changing unhealthy thought patterns which may contribute to negative moods. Both CBT and Interpersonal Therapy have been shown to improve depression in research studies.