Circadian Rhythm Sleep Disorders often involve problems with an individual’s internal clock that disrupt his or her sleep patterns; this is the case for “intrinsic” Circadian Rhythm Sleep Disorders: Delayed Phase Sleep Disorder (DSPD), Advanced Phase Sleep Disorder (ASPD), non-24, and Irregular Sleep-Wake Type (IST) Disorder. For “extrinsic” Circadian Rhythm Sleep Disorders, shift work and jet lag, there is nothing wrong with the individual’s intrinsic clock.
· Delayed Phase Sleep Disorder (DSPD): characterized by delayed sleep-onset and wake times, compared with societal or preferred norms.
· Advanced Phase Sleep Type Disorder (ASPD): characterized by habitually early sleep-onset and wake times, compared with societal or preferred norms.
· Irregular Sleep-Wake Disorder: 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.
· Non-24-Hour Sleep Wake Disorder: characterized by an abnormal sleep-wake rhythm caused by a circadian clock that is not in phase with, and cannot be entrained to, the 24-hour light-dark cycle.
· Jet Lag Disorder: occurs when there is a temporary mismatch between sleep-wake cycle timing generated by the person’s internal circadian clock and the sleep-wake pattern required by a time-zone change.
· Shift Work Disorder: characterized by insomnia or excessive sleepiness resulting from a work schedule that interferes with the normal sleep period.