Irregular Sleep-Wake Type (IST) Disorder is characterized by a lack of well-defined sleep-wake cycle; individuals with IST typically have three or more sleep episodes during a 24-hour period, rather than one major sleep period.
The total sleep time per 24-hour period is usually normal for the person's age.40
40. Reid KJ and Zee PC. “Circadian disorders of the sleep-wake cycle,” in Kryger M, Roth T, Dement W (ed.), Principles and Practice of Sleep Medicine (5th Edition), St. Louis: Elsevier Saunders, 2011, pages 470-482.