one of four basic types of sleep disorders, differentiated from the other types in that it results from a mismatch between one’s internal circadian rhythm and one’s actual sleep schedule. The equivalent classification in DSM–IV–TR is circadian rhythm sleep disorder (or in DSM–5, circadian rhythm sleep–wake disorder). Rotating work-shift schedules and jet lag are two common causes of this disorder. Diagnosis can involve observation in a sleep laboratory, in which such criteria as nocturnal awakening, sleep time, sleep efficiency, breathing patterns, body temperature, minutes of REM sleep, and sleep latency are measured.