There are several different types of depression. Two of the most common types are major depression (also called major depressive disorder) and dysthymic disorder. Other types of depression include manic depression (also known as bipolar disorder), postpartum depression and seasonal affective disorder (SAD).

Major depression is a collection of symptoms that interferes with the ability to handle daily life. Also known as clinical depression, this is what most people think of when the subject of depression comes up. The symptoms of major depression are severe--sufferers may find it nearly impossible to even get out of bed in the morning.

Dysthymia is a depression that lasts for two years or more, but with somewhat milder symptoms than major depression. Dysthymia is sometimes referred to as chronic depression. It does not disable to the extent that major depression does, but still prevents sufferers from functioning normally or working at their best. In addition, people with dysthymic disorder may also suffer episodes of major depression.

Specific symptoms of dysthymic disorder include sleep difficulties (sleeping too much or too little), lack of energy or fatigue, a feeling of hopelessness, lowered self-esteem and a difficulty making decisions or concentrating.

Because dysthymia is milder and may only involve a few symptoms, it is much more likely to go undiagnosed and untreated. The symptoms of dysthymia tend to be more enduring and are more resistant to treatment.

Manic depression (or bipolar disorder) is characterized by cycles of mania and depression. Mania refers to periods of excitement, overactivity, delusions and extreme responses. It may include violence. Manic episodes normally last for at least a week. It also includes an elevated self-esteem, less need for sleep and excessive movement. People in the midst of mania may become involved in risky, but pleasurable behavior. They are easily distracted and more talkative than usual. In some cases, the symptoms are so severe that hospitalization is required to prevent harm to self or to others.

Postpartum depression (PPD) occurs among new mothers a few months after they give birth. It is more serious and intense than the normal "baby blues" that occur in the first weeks after delivery. PPD is thought to be caused by hormone changes, but is more likely to occur in difficult situations (low partner support, a colicky baby, or history of depression).

Seasonal affective disorder, or SAD, is another form of depression. It is connected to seasonal changes, usually occurring every fall or winter. The cause of SAD is not completely understood, but it seems to be related to changes in the amount of sunlight available. Lack of sunlight may affect sleep rhythms or the balance of chemicals in the brain. Sometimes it's hard to tell the difference between seasonal affective disorder and depression. Light therapy has proven to be an effective treatment for some people, but others require antidepressant medications.