Bipolar disorder takes two principal forms, neither of which requires plural "cycles". According to the DSM-IV-TR (p. 345), these two principal forms of Bipolar disorder are:

  1. Bipolar I disorder, the diagnosis of which requires over the entire course of the individual's life at least one manic (or mixed state) episode which is usually (though not always) accompanied by episodes of Major Depressive disorder.
  2. Bipolar II disorder, which over the course of the individual's life must involve at least one Major Depressive episode and must be accompanied by at least one hypomanic episode; i.e. there need be no full manic episodes at all.
    Therefore Bipolar disorder need not have both severe mania and depression and in certain cases has only episodes of the one type. There need be no "cycles" of mania and depression.

This is the reason why certain contemporary psychiatrists shy away from the original name, Manic Depression, i.e. because the latter name might suggest that all individuals have both mania and depression. It has nothing to do with the notion of equal distribution of cycles of mania and depression, since there need not be any cycles at all--in fact, even when there is one (or more) bout of both mania and depression over the course of an individual's life, the two episodes may be so unrelated to each other temporally and otherwise that this need not constitute a cycle. However, a significant portion of individuals with bipolar experience the classical alternating episodes (cycles) of mania and depression and therefore it is overstating the case to say that the classical alternation "rarely" occurs.

The DSM-IV treats these bipolar disorders as variants of mood or affective disorders. Others types include Major Depressive Disorder and Dysthymic Disorder. Bipolar and other mood disorders may have no identifiable medical, traumatic or other external cause (endogenous) or may be due to e.g. a medical condition (exogenous).

In order for a person to be properly diagnosed with bipolar, the mood episodes cannot be due to external medication, drugs or treatment for depression.

Cycles in bipolar disorder:

Kraepelin included in his description of Manic Depression the phenomenon that episodes of acute illness, whether mania or depression, are usually punctuated by relatively symptom-free intervals during which the patient is able to function normally both at work and in social affairs.

The cycles of bipolar disorder may be long or short, and the ups and downs may be of different magnitudes: for instance, a person suffering from bipolar disorder may suffer a protracted mild depression followed by a shorter and intense mania. The manic episodes typically include euphoria, tirelessness, and impulsiveness; the depressed periods may seem much worse following a manic period.