Insomnia and excessive sleepiness are common in the investigation of sleep-disordered breathing. Circadian rhythm sleep disorders are
perhaps the most often overlooked conditions in the differential diagnosis of these symptoms. Circadian rhythm sleep disorders manifest as
misalignment between the sleep period and the physical/social 24-h environmental cycle. The two most prevalent circadian rhythm sleep
disorders are delayed sleep phase (common in adolescents) and advanced sleep phase (common in the elderly), situations in which the sleep
period is displaced to a later or earlier time, respectively. It is important to keep these two disorders in mind, since they can be confused with
insomnia and excessive sleepiness. However, there are nine possible diagnoses, and all nine are of clinical interest. Since light is the principal
cue used in synchronizing the biological clock, blind individuals and night-shift/swing-shift workers are more prone to develop circadian
rhythm sleep disorders. In this article, the new international classification of circadian rhythm sleep disorders is reviewed.
Keywords: Circadian rhythm; Sleep disorders; Sleep initiation and maintenance disorders; Sleep stages; Sleep apnea syndromes.