Circadian Rhythm Sleep-Wake Disorders 397
high frequency of mental disorders involving social isol ation and cases of non-24-hour
sleep-wake type developing after a change in sl eep habits (e.g., night shift work, job loss),
behavioral factors in combination with physi ological tendency may precipitate and per-
petuate this disorder in sighte d individuals. Hospitalized indi viduals with neurological and
psychiatric disorders can become insensitive to  social cues, predisposing them to the de-
velopment of non-24-hour sleep-wake type.
Genetic and physiological. Blindness is a risk factor for non-24-hour sleep-wake type.
Non-24-hour sleep-wake type has been as sociated with traumatic brain injury.
Diagnostic Markers
Diagnosis is confirmed by history and sleep di ary or actigraphy for an extended period.
Sequential measurement of phase markers (e.g ., melatonin) can help determine circadian
phase in both sighted and blind individuals.
Functional Consequences of 
Non-24-Hour Sleep-Wake Type
Complaints of insomnia (sleep onset and sleep  maintenance), excessive sleepiness, or both
are prominent. The unpredictability of sleep an d wake times (typically a daily delay drift)
results in an inability to attend  school or maintain a steady job and may increase potential
for social isolation. 
Differential Diagnosis
Circadian rhythm sleep-wake disorders. In sighted individuals, non-24-hour sleep-wake
type should be differentiated from delayed sl eep phase type, as individuals with delayed
sleep phase type may display a similar progressive delay in sleep period for several days. 
Depressive disorders. Depressive symptoms and depressive disorders may result in
similar circadian dysregulation and symptoms. 
Comorbidity