Ultradian cycling in bipolar disorder is a commonly used term in search of a formal definition. The term "ultradian cycling" does not appear in the Diagnostic and Statistical Manual of Mental Disorders. Some sources equate ultradian cycling with ultra-rapid cycling, which also has no official definition in the DSM-IV but, like ultradian cycling, is recognized in the psychiatric community.
Most psychiatrists use the term ultradian cycling to describe multiple mood swings in a single day.
This definition makes the most sense, in that the word "ultradian" in other contexts means happening multiple times within a day. For example, ultradian is used in reference to the cyclic changes in patterns of normal sleep throughout the night, and the waxing and waning of appetite more than once during a day.
ultra-ultra rapid cycling
ultradian rapid cycling