Depression (also known as major depression or major depressive disorder) is a psychiatric disorder that affects mood, behavior, and overall health. It causes prolonged feelings of sadness, emptiness, or hopelessness, and a loss of interest in activities that were once enjoyed. People with depression may also have changes in appetite (leading to overeating or not eating enough), changes in sleeping patterns (sleeping too much or not being able to sleep), loss of energy, and difficulty concentrating. Although depression is considered primarily a mental health disorder, it can also have physical features including headaches, other unexplained aches and pains, unusually slow or fast movements, and digestive problems. To be diagnosed with depression, an individual must have signs and symptoms nearly every day for at least 2 weeks. However, the features of this condition vary widely.
Depression most commonly begins in late adolescence or early adulthood, although it can appear at any age. If untreated, episodes of depression can last for weeks, months, or years, and can go away and come back (recur). Affected individuals may have difficulty functioning in their daily lives, including at school or work. People with depression have a higher risk of substance abuse problems and dying by suicide than the general population.
Several health conditions are closely related to depression or have depression as a characteristic feature. These include dysthymia (which has long-lasting signs and symptoms that are similar to, but not as severe as, those of depression), perinatal or postpartum depression (which occurs around or following the birth of a child), seasonal affective disorder (which is triggered by the changing of the seasons), bipolar disorder (which can include both "highs," or manic episodes, and depressive episodes), and generalized anxiety disorder. In people with schizoaffective disorder, depression or another mood disorder occurs together with features of schizophrenia (a brain disorder that affects a person's thinking, sense of self, and perceptions). [from
MedlinePlus Genetics]