Overview
As discussed in the section on sleep regulation, sleep is regulated in large part by the circadian rhythm system. This system functions as an internal biological pacemaker that ensures that we are alert during the day and sleepy at night. The circadian rhythm disorders comprise a group of sleep disorders where there is a misalignment between the timing of one's internal circadian rhythms and one's desired or required time for sleep.
This misalignment can occur because of dysfunction in the circadian rhythm system itself, as in what are known as the endogenous circadian rhythm disorders. In delayed sleep phase disorder (DSPD) circadian rhythms are delayed, such that individuals do not feel sleepy until much later at night (eg. 2am) and then desire to sleep in till much later (eg 10am). In advanced sleep phase disorder (ASPD) the circadian rhythms are advanced, such that individuals feel sleepy much earlier than normal (eg. 8pm) and then awaken earlier (eg. 4am). In free-running disorder (FRD) the circadian rhythm system runs is decoupled from the light-dark cycle causing progressively later bedtimes. Finally, irregular sleep-wake rhythm (ISWR) is characterized by a relative absence of circadian rhythms with a markedly irregular and intermittent sleep-wake pattern.
Exogenous circadian rhythm disorders are those where the circadian rhythm system is functioning normally, but a misalignment between the internal body clock and one's desired or required sleep-wake schedule occurs because of external factors. In shift work disorder (SWD) an individual is forced to keep a sleep-wake schedule that is out of phase with their internal body rhythm. In Jet Lag this misalignment is imposed by rapid travel over multiple time zones.
