Circadian rhythm sleep–wake disorder (CRSWD) is defined as sleep–wake disturbances caused by endogenous circadian system disruptions or desynchronization between internal and external sleep–wake rhythms [
60]. CRSWD patients frequently complain of chronic excessive daytime sleepiness and/or insomnia, which interferes with their activities [
60].The diagnosis of CRSWD is based on the outcome of some functional assessments that involve assessing the circadian phase using core body temperature, timing of melatonin secretion, sleep diaries, actigraphy, and subjective experiences (example, using the Morningness–Eveningness Questionnaire) [
61]. Treatment may include personalized sleep scheduling, circadian phase shifting/clock resetting, and/or the use of hypnotics and stimulant drugs [
61]. Epidemiological studies suggest that up to 3% of the global adult population experiences CRSWD [
62]. An estimated 10% of adults and 16% of adolescents with reported cases of sleep disruption may have a delay in the sleep–wake cycle of about 3–6 h later than desired [
62]. The second edition of the International Classification of Sleep Disorders classifies CRSWD as a dyssomnia, with six subtypes including advanced sleep phase, delayed sleep phase, irregular sleep–wake, free running, jet lag, and shift work types [
63]. CRSWD can also be transient (caused by jet lag, shift work, or illness) or chronic (caused by delayed sleep–wake phase disorder (DSWPD), advanced sleep–wake phase disorder (ASWPD), non-24-h sleep–wake disorder (N24SWD), and irregular sleep–wake rhythm disorder (ISWRD) [
64]. The major proven feature of all CRSWDs is the inability to fall asleep and wake up at the desired time [
64]. It is considered that CRSWDs evolve from problems with internal biological clocks and/or misalignment between the circadian timing system and the external 24-h environment [
65].
DSWPD is a common CRSWD. While widespread, it represents a small fraction of severe insomnia. Individuals who are affected report difficulty falling asleep and waking up during normal working hours [
66]. Approximately 10% of individuals with insomnia who have sought treatment in hospitals have DSWPD [
17]. Higher percentages have been reported based on surveys and telephone sampling. Integrated skewed and unbiased measures estimate the prevalence of DSWPD among the global population to be 0.13–3.1% [
67]. DSWPD is more prevalent in girls than in boys [
68].
ASWPD is characterized by persistent early evening sleep onset and early morning awakening, although the condition of awakening earlier than anticipated is common among older adults [
69]. The prevalence of ASWPD has been estimated to range from 0.25 to 7% [
70]. An advanced sleep phase phenotype was found among 0.33% of patients registered in a sleep clinic, and an estimated 1 in 2500 patients evaluated for sleep disorder has ASWPD [
71].
N24SWD is a cyclic, often devastating CRSWD characterized by severe difficulties sleeping on a 24-h schedule [
72]. Individuals isolated from a 24-h light–dark cycle exhibit sleep–wake cycles different from 24 h [
72]. N24SWD is more common among totally blind individuals because of the lack of light information reaching the circadian pacemaker in the hypothalamus [
72]. N24SWD is unusual among sighted individuals. It has been associated with delayed sleep–wake rhythm disorder or mental disorders [
72].
ISWRD is a circadian rhythm disorder characterized by multiple bouts of sleep within a 24-h period [
73]. Individuals report symptoms of insomnia, including difficulty either falling or remaining asleep, and daytime excessive sleepiness [
73]. ISWRD is associated with neurological illnesses. It is usually diagnosed among children with neurodevelopmental disorders, patients with neuropsychiatric disorders, and, most usually, older adults with neurodegenerative disorders [
73].