In the United States, 14.8 million adults suffer from major depressive disorder and 3.3 million adults suffer from dysthymic disorder, a type of chronic depression, according to the ]]>National Institute of Mental Health]]>. People who suffer from depression experience changes in mood and behavior that can interfere in their lives. For example, patients can have a regularly low mood or an irritable mood. Depression can cause patients to feel worthless or hopeless. Behavioral symptoms of depression include a loss of pleasure in activities, a lack of energy, changes in appetite, and problems sleeping. Other symptoms include problems with concentration and changes in physical movements. Some patients develop thoughts about death and suicide.
Several types of depression exist. With major depressive disorder, patients have five or more depressive symptoms over at least two weeks, though the disorder can continue for at least six months if patients do not receive treatment, according to ]]>MedlinePlus]]>. If a patient has two to four symptoms over at least two weeks, it is considered mild depression. With dysthymic disorder, patients have a low mood plus two other symptoms for at least two years. Patients who have atypical depression experience oversleeping and overeating, as well as mood reactivity. Some depression patients have their symptoms at certain times. For example, women who experience premenstrual dysphoric disorder have depressive symptoms the week before menstruation, with symptoms ending at the start of menstruation.
Different factors can contribute to depression, such as stressful life events and medical conditions, such as hypothyroidism. Genetics can also play a role in depression: in a study published in the Journal of Affective Disorder, researchers found that depression may have a link to the body's "Clock gene", which regulates the circadian clock.