Editing Somnolence (section)

=== Circadian rhythm disorders ===
{{Main|Circadian rhythm sleep disorder}}
Circadian rhythm ("biological clock") disorders are a common cause of drowsiness as are a number of other conditions such as sleep apnea, insomnia and narcolepsy.<ref name=":0"/> The body clock disorders are classified as extrinsic (externally caused) or intrinsic. The former type is, for example, shift work sleep disorder, which affects people who work nights or rotating shifts. The intrinsic types include:<ref>{{Cite web|title = Circadian Sleep Disorders Network|url = http://www.circadiansleepdisorders.org/|website = www.circadiansleepdisorders.org|access-date = 2015-11-02}}</ref>
* [[Advanced sleep phase disorder]] (ASPD) – A condition in which patients feel very sleepy and go to bed early in the evening and wake up very early in the morning
* [[Delayed sleep phase disorder]] (DSPD) – Faulty timing of sleep, peak period of alertness, the core body temperature rhythm, hormonal and other daily cycles such that they occur a number of hours late compared to the norm, often misdiagnosed as insomnia
* [[Non-24-hour sleep–wake disorder]] – A faulty body clock and sleep-wake cycle that usually is longer than (rarely shorter than) the normal 24-hour period causing complaints of insomnia and excessive sleepiness
* [[Irregular sleep–wake rhythm]] – Numerous naps throughout the 24-hour period, no main nighttime sleep episode and irregularity from day to day