396 Sleep-Wake Disorders
Comorbidity
Irregular sleep-wake type is often comorbid  with neurodegenerat ive and neurodevelop-
mental disorders, such as major neurocognitive disorder, inte llectual disability (intellec-
tual developmental disorder), and traumatic br ain injury. It is also  comorbid with other
medical conditions and mental disorders in wh ich there is social isolation and/or lack of
light and structured activities. 
Non-24-Hour Sleep-Wake Type
Diagnostic Features
The diagnosis of non-24-hour sleep-wake type  is based primarily on a history of symp-
toms of insomnia or excessive sleepiness related to abnormal synchronization between the
24-hour light-dark cycle and the endogenous circadian rhythm. Individuals typically pre-
sent with periods of insomnia, excessive sleepiness, or both, which alternate with short
asymptomatic periods. Starting with the asymptomatic period, when the individual’s
sleep phase is aligned to the external envi ronment, sleep latency will gradually increase
and the individual will complain of sleep-onset insomnia. As the sleep phase continues to
drift so that sleep time is no w in the daytime, the individu al will have trouble staying
awake during the day and will complain of sleepiness. Because the circadian period is not
aligned to the external 24-hour environment,  symptoms will depend on when an individ-
ual tries to sleep in relation to the circadian rhythm of sleep propensity. 
Associated Features Supporting Diagnosis
Non-24-hour sleep-wake type is most common among blind or visually impaired individ-
uals who have decreased light perception. In si ghted individuals, there is often a history of
delayed sleep phase and of decreased exposure to light and structured social and physical
activity. Sighted individuals with non-24-h our sleep-wake type also demonstrate in-