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.

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:

• Adolescent depression
• Depression in the elderly

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:

• Trouble sleeping or excessive sleeping
• A dramatic change in appetite, often with weight gain or loss
• Fatigue and lack of energy
• Feelings of worthlessness, self-hate, and inappropriate guilt
• Extreme difficulty concentrating
• Agitation, restlessness, and irritability
• Inactivity and withdrawal from usual activities
• Feelings of hopelessness and helplessness
• Recurring thoughts of death or suicide

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.

Depressed children may not have the classic symptoms of adult depression. Watch especially for changes in school performance, sleep, and behavior. If you wonder whether your child might be depressed, it's worth bringing to a doctor's attention.

The main types of depression include:

• Major depression -- five or more symptoms listed above must be present for at least 2 weeks, but major depression tends to continue for at least 6 months. (Depression is classified as minor depression if you have fewer than five depression symptoms for at least 2 weeks. In other words, minor depression is similar to major depression except it only has 2 - 4 symptoms.)
• Atypical depression -- occurs in about a third of patients with depression. Symptoms include overeating and oversleeping. These patients tend to have a feeling of being weighed down and react strongly to rejection.
• Dysthymia -- a generally milder form of depression that lasts as long as 2 years.

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 (PMDD) -- depressive symptoms occur 1 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 due to your genes (inherited), learned behavior, or both. Even if your genes make you more likely to develop depression, a stressful or unhappy life event usually triggers the onset of a depressive episode.

Depression may be brought on by:

• Alcohol or drug abuse
• Childhood events like abuse or neglect
• Chronic stress
• Death of a friend or relative
• Disappointment at home, work, or school (in teens, this may be breaking up with a boyfriend or girlfriend, failing a class, or parents divorcing)
• Drugs such as sedatives and high blood pressure medications
• Medical conditions such as hypothyroidism (underactive thyroid), cancer, or hepatitis
• Nutritional deficiencies (such as a lack of folate and omega-3 fatty acids)
• Overly negative thoughts about one's self and life, self blame, and ineffective social problem solving skills
• Prolonged pain or having a major illness
• Sleeping problems
• Social isolation (common in the elderly)