It is common for a person to feel sad, down, or blue from time to time, but if this condtion persists or is recurrent it is possible that the person is suffering from clinical depression. Common symtoms of clinical depression include:

• feelings of overwhelming sadness, or seeming unable to feel emotion
• marked decrease of interest in pleasurable activities
• changing appetite and marked weight gain or weight loss
• disturbed sleep patterns, either insomnia or sleeping more than normal 
•  changes in activity levels, restless or moving significantly slower than normal
• fatigue, both mental and physical 
•  feelings of guilt, helplessness, anxiety, and/or fear
• lowered self-esteem
• decreased ability to concentrate or make decisions

Treatment of depression varies broadly, and is different for each individual. Various types and combinations of treatments may have to be tried. There are two primary modes of treatment, typically employed in conjunction with one another, medication and psychotherapy. A third treatment, electroconvulsive therapy (ECT) may be used where chemical treatment fails. Other alternative treatments used for depression include exercise, and the use of vitamins, herbs, or other nutritional supplements.