similar circadian dysregulation and symptoms. 
Comorbidity
Blindness is often comorbid with non-24-hour sleep-wake type, as are depressive and bi-
polar disorders with social isolation.
Shift Work Type
Diagnostic Features
Diagnosis is primarily based on a history of th e individual working outside of the normal
8:00 A.M. to 6:00 P.M. daytime window (particularly at ni ght) on a regularly scheduled (i.e.,
non-overtime) basis. Symptoms of excessive sleepiness at work, and impaired sleep at
home, on a persistent basis are prominent. Pres ence of both sets of symptoms are usually
required for a diagnosis of shift work type. Ty pically, when the individual reverts to a day-
work routine, symptoms resolve. Although the etiology is slightly different, individuals
who travel across many time zones on a very frequent basis may experience effects similar
to those experienced by individuals with shift work type who work rotating shifts. 
Prevalence
The prevalence of shift work ty pe is unclear, but the disorder is estimated to affect 5%–10%
of the night worker population  (16%–20% of the workforce). Prevalence rises with advance-
ment into middle-age and beyond (Drake et al. 2004).