ABSTRACT
Circadian rhythm sleep disorders (CRSDs) are a family of disorders that include problems with the timing of sleep and resultant daytime or nighttime symptoms. The term “circadian rhythm” refers to the endogenous rhythm of slightly greater than 24 hours, which has an important influence over many biological and physiological processes. Circadian variation has been recognized and recorded in living things since ancient times. Though endogenously generated, it may be modified or entrained by external environmental cues. Human circadian rhythms, including sleep-wake cycles, are driven by the suprachiasmatic nucleus (SCN) of the anterior hypothalamus. The SCN responds to light and non-light cues to synchronize the body’s internal rhythms with the external world. The near-24 hour rhythm of the SCN is normally entrained to the 24-hour light-dark cycle of the earth’s rotation around the sun (1). Melatonin secretion from the pineal gland, which is driven by the SCN, peaks after dark onset and is linked to sleep propensity (1). Light in the evening causes a rapid suppression of melatonin concentrations (2,3).
