“normal” sleep patterns in which an individual has a late schedule that does not cause
personal, social, or occupational distress (m ost commonly seen in adolescents and young
adults).
Other sleep disorders. Insomnia disorder and other circadian rhythm sleep-wake dis-
orders should be included in the differenti al. Excessive sleepiness may also be caused by
other sleep disturbances, such as breathing- related sleep disorders, insomnias, sleep-
related movement disorders, and medical, ne urological, and mental disorders. Overnight
polysomnography may help in evaluating for other comorbid sleep disorders, such as
sleep apnea. The circadian nature of delaye d sleep phase type, howe ver, should differen-
tiate it from other disorders with similar complaints.
Comorbidity
Delayed sleep phase type is strongly associated  with depression, personality disorder, and
somatic symptom disorder or illness anxiety d isorder. In addition, comorbid sleep disor-
ders, such as insomnia disorder, restless legs syndrome, and sleep apnea, as well as depres-
sive and bipolar disorders and anxiety disord ers, can exacerbate symptoms of insomnia
and excessive sleepiness. Delayed sleep phase  type may overlap with another circadian
rhythm sleep-wake diso rder, non-24-ho ur sleep-wake type. Sighted individuals with non-
24-hour sleep-wake type disorder commonly also have a history of delayed circadian sleep
phase.