Circadian Rhythm Sleep Disorders Sun Jung Han, Eun Yeon Joo Department of Neurology, Sanbon Medical Center, Wonkwang University School of Medicine Department of Neurology, Sleep Center, Samsung Medical Center, Sungkyunkwan University School of Medicine, Seoul Circadian rhythm sleep disorders are characterized by an alteration of the phase relationship between the intrinsic circadian system and the extrinsic lightdark cycle that results in complaints of insomnia and excessive sleepiness. Under entrainment to the 24-hour day-night cycle, the sleep-wake pattern may assume various phase relationships to the circadian pacemaker by abnormally long or short intervals between the dim light melatonin onset (DLMO) or the core body temperature (peak time) and wake-up time. Delayed and advanced sleep phase disorders may be associated with shifted circadian rhythms (body clock). Those with ‘weak’ circadian rhythms need a very bright stimulus of light to reset their body clock every day. Treatments include sleep hygiene education, timed exposure to bright light as well as avoidance of bright light at the wrong time of the day and pharmacologic approaches, such as melatonin. This article introduces the clinical approach to the diagnosis and management of the various circadian rhythm sleep disorders, including delayed and advanced sleep phase disorders, shift worker, and jet lag.