I have experienced chronic depression for most of my life and additionally a type of “rapid cycling” that is dismissed for the most part by psychiatrists. My “rapid cycling” seemed to often occur with in one day(extreme diurnal variation) and over three or more days, almost a cycling within cycling. The cycling often appeared out of a substrate of chronic depression and the timing of the cycling was almost something one could set their watch to. The extent of the mood switch was considerable, from a suicidal depression to a rather productive euthymic/hypomanic mood. To this day I am not sure what precipitated it.
PsychEducation is one site that discusses the possibility of cycles occurring within “unipolar” depression. This site also mentions and demonstrates the possibility of mixed states occurring within various cycles when depression is subcatorgized into mood, activity, and intellect. One can, for example, have a low mood plus high activity which could equal irritability in many.
But patients do have other combinations of depression and hypomania, or mania — not just the two worst phases together. And they do have cycles shorter than 4 days. The DSM can’t really handle these variations, but the model shown here handles them very well.
Additionally, during this “cycling” the purity of my depression would in turn effect the purity of the euthymia/hypomania. A “pure” depressive mood which meant low irritability/anxiety would often be accompanied, within the same day, by a pleasant and productive euthymic state. The amplitude of the cycle was often predicted by the depression. Prior to unusually high mood a unusually low depressive dip would often occur. Incidently one of the most severe cycles occurred while taking Lithium.
I firmly believe that this cycling I experienced was unusual/abnormal however I am not certain that it suggests bipolarity. On the other hand it seems highly unlikely to me that on a molecular level, unipolar depression is dramatically different than bipolar disorder. I see the generally principle of “for every reaction there is an equal and opposite one.”, opporating in both disorders.