The aim of this review is to assess the typical time of day that mood switches occur in ultra-rapid cycling disorder. It shall be determined, whether switches into depression or (hypo)mania are likelier to occur at particular times of the day or night, and whether a pattern of mood switch times can be discerned for different cycle lengths. Case reports giving information about cycle lengths and the times of day when switches have occurred were systematically compiled (sources: Medline and Web of Science). Cases with ultra-rapid (cycle lengths of days to weeks, including 48-h cycling) or ultra-ultra-rapid cycling (cycle lengths up to 24 h) were included (35 publications with 42 case reports). In cases with cycle lengths of days to weeks (n=11), switches appeared to occur at any time of the day. Patients with regular 48-h cycling (n=28) switched to depression or mania most often during nighttime hours. In all three patients with cycle lengths up to 24 h (ultra-ultra-rapid cycling), switches into depression occurred at night and switches into mania occurred at daytime. These findings indicate that in patients with mood cycles of 48h or less the switch process is closely linked to circadian aspects.
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