Feelings of depression may be described as feeling sad, blue, unhappy, miserable, or down in the dumps. Most of us feel this way at one time or another for short periods. But true clinical depression is a mood disorder in which feelings of sadness, loss, anger, or frustration interfere with everyday life for an extended period of time.

See also depression in the elderly and adolescent depression.

Depression is generally ranked in terms of severity -- mild, moderate, or severe. The degree of your depression, which your doctor can determine, influences how you are treated. Symptoms of depression include:

• Sleep disturbances -- usually insomnia (for example, consistently waking up very early in the morning) but may be excessive sleeping
• A dramatic change in appetite, often resulting in either weight gain or weight loss
• Fatigue and lack of energy
• Feelings of worthlessness, self-hate, and inappropriate guilt
• Extreme difficulty concentrating
• Agitation, restlessness, and irritability OR inactivity and withdrawal from usual activities
• Recurring thoughts of death or suicide
• Feelings of hopelessness and helplessness

Low self esteem is common with depression. So are sudden bursts of anger and lack of pleasure from activities that normally make you happy, including sex.

The main types of depression include:

• Major depression -- five or more symptoms listed above must be present for at least 2 weeks, but tends to continue for 20 weeks. (A mood disorder is classified as minor depression if less than five depressive symptoms are present for at least 2 weeks.)
• Dysthymia -- a chronic, generally milder form of depression but lasts longer -- usually as long as two years.
• Atypical depression -- depression accompanied by unusual symptoms, such as hallucinations (for example, hearing voices that are not really there) or delusions (irrational thoughts).

Other common forms of depression include:

• Postpartum depression -- many women feel somewhat down after having a baby, but true postpartum depression is rare.
• Premenstrual dysphoric disorder (PDD) -- depressive symptoms occur one week prior to menstruation and disappear after you menstruate.
• Seasonal affective disorder (SAD) -- occurs during the fall-winter season and disappears during the spring-summer season. Likely to be due to lack of sunlight.

Depression may also occur with mania (known as manic-depression or bipolar disorder). In this condition, moods cycle between mania and depression.

Depression is more common in women than men and is especially common during the teen years. Men seem to seek help for feelings of depression less often than women. Therefore, women may only have more documented cases of depression.

Depression often runs in families. This may be from heredity, learned behavior, or both. Even with a genetic predisposition, it is usually a stressful or unhappy life event that triggers the onset of a depressive episode.

Depression may be brought on by:

• Disappointment at home, work, or school. In teens for example, this may be breaking up with a boyfriend or girlfriend, failing a class, or parents divorcing.
• Death of a friend or relative.
• Prolonged pain.
• Medical conditions such as hypothyroidism (under-active thyroid), cancer, or hepatitis.
• Drugs such as sedatives for sleep or anxiety and certain high blood pressure medications.
• Alcohol or drug intoxication or alcohol or drug withdrawal.

Other factors that can contribute to feelings of depression include chronic stress (including childhood events like abuse or neglect), social isolation (common in the elderly), nutritional deficiencies (such as folate and omega-3 fatty acids), sleep disturbances, cigarette addiction, and little exposure to daylight.