For major depression, you may experience a sad or depressed mood, or an inability to feel pleasure, plus five or more of the following symptoms, for at least a two-week period

• Feelings of guilt, worthlessness, helplessness, or hopelessness

• Loss of interest or pleasure in usual activities, including sex

• Difficulty concentrating and complaints of poor memory

• Insomnia or oversleeping

• Appetite changes, which may include weight gain or loss

• Fatigue, lack of energy

• Thoughts of suicide or death

• Slow speech; slow movements

• Overeating

• Oversleeping

• Fatigue

• Extreme sensitivity to rejection

• Moods that worsen or improve in direct response to events

• Regular -- or "typical" -- depression, on the other hand, tends to be marked by pervasive sadness and a pattern of loss of appetite and difficulty fall or staying asleep.

Major Depressive Disorder

According to the National Institute of Mental Health, major depressive disorder is characterized by a combination of symptoms that interfere with a person's ability to work, sleep, study, eat, and enjoy once pleasurable activities. There are times you may feel sad, lonely, or hopeless for a few days. But major depression -- clinical depression -- lasts longer and is disabling. It can prevent you from functioning normally. An episode of clinical depression may occur only once in a person's lifetime. More often, though, it recurs throughout a person's life. In addition, with major depression, one of the symptoms must be either depressed mood or loss of interest. The symptoms should be present daily or for most of the day or nearly daily for at least two weeks. Also, the depressive symptoms must cause clinically significant distress or impairment in functioning. The symptoms cannot be due to the direct effects of a substance -- drug abuse, medications -- or a medical condition, such as hypothyroidism, nor occur within two months of the loss of a loved one.