This chapter discusses the symptom of sleepiness in two of the most common circadian rhythm sleep disorders: advanced sleep phase disorder (ASPD) and delayed sleep phase disorder (DSPD). The personality disorders are diagnosed more frequently in those with DSPD compared with other sleep disorder populations, including ASPD. In ASPD, an individual's circadian sleep tendency is advanced relative to the (societally) desired clock time. A number of investigators have recently reported age-related changes in the circadian timing system that may be associated with the sleep patterns often observed in older subjects. The effective use of current interventions in the treatment of DSPD and ASPD, as well as the development of new approaches, depends largely on the findings obtained from this type of analytic approach. This is emphasized by the results of several studies showing successful phase shifts of the circadian system, without any improvement in sleep quality.