For most people the period of depression is brief and has no significant effect on their activities, however clinical depression is more serious and can become debilitating. Clinical depression is characterized by feelings of sadness, disappointment, loneliness, hopelessness, self doubt, and guilt that lasts for more than two weeks. This depressed state makes activity difficult, and if the depression is severe it can result in the person completely withdrawing from the world. Depression may be triggered by a specific event in a person's life, such as loss of a loved one or loss of a job, but often depression has no recognizable cause. It is thought that the tendency toward some types of depression is inherited.
Depression is associated with an imbalance of chemicals in the brain.
The brain is made up of a network of nerve cells called neurons. Messages are transmitted from one neuron to another by chemicals called neurotransmitters. In cases of depression, the neurotransmitters serotonin and norepinephrine may not be produced in sufficient quantities. The result is that too few messages are transmitted and the symptoms of depression occur. It is believed that a lack of serotonin causes sleep problems, irritability and anxiety, and a lack of norepinephrine causes fatigue, feelings of hopelessness, etc.