activity. Sighted individuals with non-24-h our sleep-wake type also demonstrate in-
creased sleep duration.
Prevalence
Prevalence of non-24-hour sleep-wake type in the general population is unclear, but the
disorder appears rare in sighte d individuals. The prevalence in blind individuals is esti-
mated to be 50%.
Development and Course
Course of non-24-hour sleep-wake type is per sistent, with intermittent remission and ex-
acerbations due to changes in work and social schedules throughout the lifespan. Age at
onset is variable, depending on the onset of visual impairment. In sighted individuals, be-
cause of the overlap with delayed sleep phase  type, non-24-hour sleep-wake type may de-
velop in adolescence or early adulthood. Remi ssion and relapse of symptoms in blind and
sighted individuals largely depend on adhere nce to treatments designed to control sleep
and wake structure and light exposure.
Clinical expression may vary across the lif espan depending on social, school, and work
obligations. In adolescents and adults, irre gular sleep-wake schedules and exposure to
light or lack of light at critical times of the day can exacerbate the effects of sleep loss and
disrupt circadian entrainment. Consequently , symptoms of insomnia, daytime sleepiness,
and school, professional, and interpersonal functioning may worsen.
Risk and Prognostic Factors
Environmental. In sighted individuals, decreased expo sure or sensitivit y to light and so-
cial and physical activity cues may contribute to a free-running circadian rhythm. With the