Depression is a common and serious mood disorder that negatively affects how you feel, the way you think, and the way you act. It is not the same as being unhappy or in a blue mood. Depression affects how you sleep, eat, work, study, and interact with others and can lead to suicide. Depression can last for a few weeks or longer and can vary from mild to severe. Even though it causes pain and impairment, depression is highly treatable. Treatments include medication, therapy, and lifestyle changes.