Major depressive episodes are characterized by five or more of the following symptoms, all of which must be present in a more or less uninterrupted manner for at least a two week period:
- A pervasive depressed mood that colors and tones daily experience
- A diminished ability to take pleasure from activities that used to be pleasurable (such as sex, food, hobbies, social interaction)
- Appetite changes (either more hungry or less), which may be accompanied by weight loss or gain. (No conscious dieting is occurring)
- Sleep changes (either sleeping more or less than normal)
- Psychomotor (e.g., body) agitation or retardation; either can't sit still, or can hardly move.
- Constant complaints of fatigue and low energy
- Thoughts of the affected person's worthlessness, guilt or shame plague him or her
- Concentration becomes more difficult to achieve than before
- Thoughts of the desirability of death and suicide
People experiencing a major depressive episode may be lacking in energy and show slower, unmotivated movements, or they may appear irritable and agitated. They may have a hard time getting out of bed in the morning, or they may stay up all night with insomnia. Either way, they are likely to complain of constant tiredness and difficulty concentrating on tasks. They may eat very little or eat to excess so as to comfort themselves, possibly leading to rapid changes in weight. They may lose interest in doing things they previously enjoyed or spending time with other people. The low mood tone, inability to accomplish tasks, and general shut-down of the brain's ability to think clearly and rationally can lead to exaggerated feelings of worthlessness, misery and despair. Such extreme negative feelings and self-judgments drive a substantial minority of bipolar-diagnosed people towards contemplating and (all too often) actually committing suicide.
There is no corollary to a hypomanic episode for depression; no short-term "hypodepressive" episode that can be diagnosed. There is a related condition known as Dysthymic Disorder or Dysthymia, which describes a long-lasting mild depression. Dysthymia cannot be diagnosed at the same time as bipolar disorder, however, because in order to qualify for a diagnosis of Dysthymia, you have to show evidence of consistently mild depressive symptoms occurring more days than not over a period of at least two years. The presence of manic or hypomanic episodes during the two year period would disqualify any dysthymic disorder diagnosis.
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While bipolar disorder most frequently manifests as a swing between manic and depressive episodes, in a minority of cases, a third type of Mixed mood episode occurs. In a mixed episode, the criteria for mania and the criteria for depression are both simultaneously met more often than not for at least a one week period duration. Just because criteria for both manic and depressive episodes are both met during a single day does not mean that both sets of symptoms are simultaneously present, however. Instead, what appears to be more the case is that there is a rapid alteration between manic and depressive states, occurring one or more times in a single day. Mixed episodes tend to be severe when they occur; psychotic symptoms such as hallucinations and delusions, and suicidal thinking are frequently present.