The paradoxical effect of sleep deprivation in endogenous depression has renewed the traditional interests of psychiatrists in the rhythmic phenomena associated with the symptomatology and course of cyclothymia. During the last decade, the rapid development of clinical research on sleep, neuroendocrinology, chromobiology and chronopathology in conjunction with important developments of methodology and statistical procedures have enabled a modern reconceptualization of the relationship between cyclothymia and disturbances in biological rhythms. An intensive review of the literature on circadian, ultradian, and infradian rhythms is reported here, and their relationship to the course and individual phases of cyclothymia is explored. The results of traditional analyses of sleep stages which had originally indicated that particular disturbances of sleep may be specific to cyclothymia have been shown to be incorrect. Rather, the abbreviation and fragmentation of sleep and the decrease of del asleep with concomitant increase in shallow stages have been found as well in serious sleep disturbances of other origins. Striking inter-and intra-individual variabilities in REM-sleep parameters characterize not only psychotic depression and severe mania but also a wide range of other acute psychoses...