Biological Causes

Research suggests that people suffering from depression have imbalances of neurotransmitters, natural substances that allow brain cells to communicate with one another. Two transmitters implicated in depression are serotonin and norepinephrine. Scientists think a deficiency in serotonin may cause the sleep problems, irritability, and anxiety associated with depression. Likewise, a decreased amount of norepinephrine, which regulates alertness and arousal, may contribute to the fatigue and depressed mood of the illness.

Other body chemicals also may be altered in depressed people. Among them is cortisol, a hormone that the body produces in response to stress, anger, or fear. In normal people the level of cortisol in the bloodstream peaks in the morning, then decreases as the day progresses. In depressed people, however, cortisol peaks earlier in the morning and does not level off or decrease in the afternoon or evening.

Researchers don't know if these imbalances cause the disease or if the illness gives rise to the imbalances. They do know that cortisol levels will increase in anyone who must live with long-term stress.