Unlike mania, depression is something many people experience at some point in their life. Therefore the term depression covers a very wide range of experiences. At the most mild end of the spectrum are transient feelings of sadness. Longer periods of sadness following losses or other upsetting events also fall within the normal range of human experience. When feelings of sadness become the constant backdrop of a person’s mood, we are moving into depression.

While feelings of depression following difficult life experiences are still very common, severe depression is markedly different from these milder and more transient types of depression. DSM-IV refers to the most severe form of depression as a major depressive episode. To meet criteria for a major depressive episode, the individual has to exhibit at least five of the following symptoms over a two-week period and the symptoms have to represent a change from the person’s previous state. These symptoms include: consistently depressed mood, diminished interest in activities, significant weight gain or loss (not due to dieting), increase or decrease in sleep (insomnia or hypersomnia), physical restlessness or slowing down (psychomotor agitation or retardation), loss of energy, feelings of worthlessness and guilt, and thoughts of death or suicide. When someone has one or more of these episodes, they are diagnosed with major depressive disorder—presuming the episodes cannot be attributed to another mental disorder such as bipolar disorder or a substance-induced depression.