ABSTRACT
INTRODUCTION Circadian rhythm sleep disorders (CRSDs) arise from either an alteration in the function of the circadian timing system or a misalignment between the circadian rhythm of sleep propensity and the requirements of the environmental or socially structured sleep schedule. There are six recognized CRSDs: (i) delayed sleep phase disorder (DSPD), (ii) advanced sleep phase disorder (ASPD), (iii) irregular sleep-wake disorder (ISWD), (iv) free-running disorder (FRD), also called nonentrained or non-24-hour sleep-wake disorder, (v) jet lag disorder (JLD), and (vi) shift work disorder (SWD). Insomnia (difficulty falling asleep or staying asleep) is a major symptom of all the CRSDs, as well as sleepiness and dysphoria while awake. To meet the full diagnostic criteria for a sleep disorder, the symptoms must be persistent and involve a significant impairment in social, occupational, or other areas of function.
