in DSM–IV–TR, a sleep disorder that is due to a mismatch between the sleep–wake schedule required by a person’s environment or occupational circumstances (e.g., shift work) and his or her circadian sleep–wake pattern, resulting in excessive sleepiness or insomnia. This disorder was formerly called sleep–wake schedule disorder. DSM–5 calls it circadian rhythm sleep–wake disorder. See dyssomnia. See also disorders of the sleep–wake cycle schedule.