According to the definitions of most medical, psychological and psychiatric bodies, there is a commonality in the diagnosis of depression. Most depression tests have a very similar framework. Almost without exception, clinical depression will be diagnosed if a certain number of feelings, that are signs of depression, are present over a certain period of time.

Current theory suggests those clinical depression results from complex interactions between brain chemicals and hormones that influence a person's energy level, feelings, sleeping and eating habits. These chemical interactions are linked to many complex causes--a person's family history of illness, biochemical and psychological make-up, prolonged stress, and traumatic life crisis such as death of a loved one, job loss, or divorce. Following are some of the common symptoms of clinical depression:

Markedly diminished interest or pleasure in all, or almost all, activities most of the day, nearly every day.

Significant weight loss when not dieting or weight gain, or decrease or increase in appetite nearly every day. Note: In children, consider failure to make expected weight gains.

Insomnia or hypersomnia nearly every day

Psychomotor agitation or retardation nearly every day

Fatigue or loss of energy nearly every day

Feelings of worthlessness or excessive or inappropriate guilt (which may be delusional) nearly every day.

Exhaustion on waking

Disrupted sleep, sometimes through upsetting dreams

Early morning waking and difficulty getting back to sleep

Doing less of what they used to enjoy

Difficulty concentrating during the day

Improved energy as the day goes on

Anxious worrying and intrusive upsetting thoughts

Becoming emotional or upset for no particular reason

Shortness of temper, or irritability

Restlessness or decreased activity noticed by others

Thoughts of suicide or death

Teenagers may show depression by taking more risks and/or showing less concern for their own safety.