Non-24-hour sleep-wake type is most common among blind or visually impaired individ- uals who have decreased light perception. In sighted 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-hour sleep-wake type also demonstrate in- creased sleep duration.
Prevalence of non-24—hour sleep-wake type in the general population is unclear, but the disorder appears rare in sighted individuals. The prevalence in blind individuals is esti- mated to be 50%.
Course of non-24-hour sleep—wake type is persistent, 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. Remission and relapse of symptoms in blind and sighted individuals largely depend on adherence to treatments designed to control sleep and wake structure and light exposure.
Clinical expression may vary across the lifespan depending on social, school, and work obligations. In adolescents and adults, irregular 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.
Environmental. In sighted individuals, decreased exposure or sensitivity to light and so- cial and physical activity cues may contribute to a free-running circadian rhythm. With the high frequency of mental disorders involving social isolation and cases of non-24—hour sleep-wake typexdeveloping after a change in sleep habits (e.g., night shift work, job loss), behavioral factors in combination with physiological tendency may precipitate and per- petuate this disorder in sighted individuals. Hospitalized individuals 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 associated with traumatic brain injury.
Diagnosis is confirmed by history and sleep diary 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.
Complaints of insomnia (sleep onset and sleep maintenance), excessive sleepiness, or both are prominent. The unpredictability of sleep and 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.
Circadian rhythm sleep-wake disorders. In sighted individuals, non-24—hour sleep-wake type should be differentiated from delayed sleep 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.
Blindness is often comorbid with non-24—hour sleep-wake type, as are depressive and bi- polar disorders with social isolation.
Diagnosis is primarily based on a history of the individual working outside of the normal 8:00 AM. to 6:00 PM. daytime window (particularly at night) 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. Presence of both sets of symptoms are usually required for a diagnosis of shift work type. Typically, when the individual reverts to a day- work routine, symptoms resolve. Although the etiology is slightly different, individuals to those experienced by individuals with shift work type who work rotating shifts.