The following is an article from the National Institutes of Mental Health.

What Is Depression?

Everyone occasionally feels blue or sad, but these feelings are usually fleeting and pass within a couple of days. When a person has a depressive disorder, it interferes with daily life, normal functioning, and causes pain for both the person with the disorder and those who care about him or her. Depression is a common but serious illness, and most who experience it need treatment to get better.

Many people with a depressive illness never seek treatment. But the vast majority, even those with the most severe depression, can get better with treatment. Intensive research into the illness has resulted in the development of medications, psychotherapies, and other methods to treat people with this disabling disorder.

What are the signs and symptoms of depression?

People with depressive illnesses do not all experience the same symptoms. The severity, frequency and duration of symptoms will vary depending on the individual and his or her particular illness.

Symptoms include:

• Persistent sad, anxious or “empty” feelings
• Feelings of hopelessness and/or pessimism
• Feelings of guilt, worthlessness and/or helplessness
• Irritability, restlessness
• Loss of interest in activities or hobbies once pleasurable, including sex
• Fatigue and decreased energy
• Difficulty concentrating, remembering details and making decisions
• Insomnia, early–morning wakefulness, or excessive sleeping
• Overeating, or appetite loss
• Thoughts of suicide, suicide attempts
• Persistent aches or pains, headaches, cramps or digestive problems that do not ease even with treatment

What causes depression?

There is no single known cause of depression. Rather, it likely results from a combination of genetic, biochemical, environmental, and psychological factors.

Research indicates that depressive illnesses are disorders of the brain. Brain-imaging technologies, such as magnetic resonance imaging (MRI), have shown that the brains of people who have depression look different than those of people without depression. The parts of the brain responsible for regulating mood, thinking, sleep, appetite and behavior appear to function abnormally. In addition, important neurotransmitters–chemicals that brain cells use to communicate–appear to be out of balance. But these images do not reveal why the depression has occurred.

Some types of depression tend to run in families, suggesting a genetic link. However, depression can occur in people without family histories of depression as well. Genetics research indicates that risk for depression results from the influence of multiple genes acting together with environmental or other factors.

In addition, trauma, loss of a loved one, a difficult relationship, or any stressful situation may trigger a depressive episode. Subsequent depressive episodes may occur with or without an obvious trigger.

Please visit the National Institutes of Mental Health website at http://www.nimh.nih.gov for more information.