Circadian Rhythm Sleep-Wake Disorders 395
circadian rhythm. There is no major sleep period, and sleep is fragmented into at least
three periods during the 24-hour day. 
Associated Features Supporting Diagnosis
Individuals with irregular slee p-wake type typically present with insomnia or excessive
sleepiness, depending on the time of day. Sleep and wake periods across 24 hours are frag-
mented, although the longest sleep period tends to occur between 2:00 A.M. and 6:00 A.M.
and is usually less than 4 hours. A history of isolation or reclusion may occur in association
with the disorder and contribute to the symptoms  via a lack of external stimuli to help en-
train a normal pattern. Individuals or their ca regivers report frequent  naps throughout the
day. Irregular sleep-wake type is most comm only associated with neurodegenerative dis-
orders, such as major neurocognitive disorder, and many neurodevelopmental disorders
in children. 
Prevalence
Prevalence of irregular sleep-wake type in the general population is unknown. 
Development and Course
The course of irregular sleep-wak e type is persistent. Age at onset is variable, but the dis-
order is more common in older adults. 
Risk and Prognostic Factors
Temperamental. Neurodegenerative disorders, such as  Alzheimer’s disease, Parkinson’s
disease, and Huntington’s disease, and neur odevelopmental disorders in children in-
crease the risk for irregular sleep-wake type. 
Environmental. Decreased exposure to environmenta l light and structured daytime ac-
tivity can be associated with a low-amplitud e circadian rhythm. Hospitalized individuals
are especially prone to such weak external en training stimuli, and even outside the hospi-