The article offers an overview of the sleep disorders along with brief descriptions of the most recent developments advancing the field. This area of medicine has experienced a quasi-explosive expansion since the early development of clinical sleep laboratories in the 1970s. Sleep medicine oversees conditions as common as insomnia, restless legs syndrome, and sleep apnea, which have emerged as important risk factors for vascular disease and other ailments such as diabetes. New information points to moderate to severe sleep apnea as a factor in cognitive decline. Sleep neurology also covers less common but equally socially destructive disorders such as narcolepsy and the circadian dysrhythmias. Many neurologic disorders have associated sleep dysfunctions. The study of sleep physiology and pathophysiology is a window to the function of the brain. Overall successful clinical management of sleep disorders, whether breathing disorders, insomnias, hypersomnias, or parasomnias, improves the quality of life. This article discusses the most salient innovations and discoveries in sleep medicine that have occurred during the preceding year.
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• Sleep medicine oversees conditions as common as insomnia, restless legs syndrome, and sleep apnea that reduce the quality of life and diminish the health of the individual.
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• Some of these conditions have emerged as important risk factors for vascular disease, diabetes, and cognitive decline.
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• It also covers less common but equally socially destructive disorders such as narcolepsy and the circadian dysrhythmias.
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• Many neurologic disorders have associated sleep dysfunctions.
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• The study of sleep physiology and pathophysiology is a window to the function of the brain.
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• Successful clinical management of sleep disorders, whether insomnias, hypersomnias, parasomnias, or breathing disorders, improves the quality of life and ameliorates the health of the individual.
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• Sleep dysfunctions may underlie modifiable cognitive degradation.
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• Insomnia may have deleterious consequence if persistent and pervasive.
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• The current COVID-19 pandemic has decompensated various sleep disorders.
Historical note and terminology
Although sleep is a universal experience, it was not studied systematically by scientists and physicians until the 20th century. Prior to the discovery of brain electrical activity, sleep was often assumed to be a passive response intended to rest during reduced cerebral stimulation associated with mental and cerebral inactivity. Aristotle believed that the condition of sleep was initiated by warm vapors from the stomach (48). Others attributed sleep to the effects of vascular congestion or anemia or to the buildup of "hypnotoxins." Sleepiness, dreaming, and cataplexy are eloquently displayed in Dante Aliguieri’s (1265-1321), Divine Comedy, a journey into the afterlife (87). Franz Kafka was obsessed and plagued with insomnia, delayed sleep phase disorder, inadequate sleep hygiene, and chronic sleep deprivation (56).
Berger's demonstration of changes in the electroencephalogram (EEG) during sleep followed by the sleep-EEG studies of Loomis and colleagues provided the first definite evidence that the brain is not passive during sleep (16; 69). Studies of encephalitis lethargica by von Economo, of hypothalamic and thalamic stimulation by Hess, and of the reticular activating system by Moruzzi and Magoun provided major advances in the understanding of the neuroanatomical substrate of sleep and wakefulness (115; 47; 77). The discoveries of rapid eye movement (REM) sleep (08) and repetitive cycles of REM and NREM sleep throughout the night (35) led to a new view of sleep as an active process with distinctive neurophysiological substrates underlying the 2 major sleep states.
Physicians have known for centuries that sleep disturbance is often a sign of disease, but the recognition that primary sleep disorders are common, serious, and often treatable has occurred mainly in the second half of the 20th century. Narcolepsy, recognized in the 19th century, was often considered a form of epilepsy or a psychiatric disturbance until the discovery of its association with abnormal REM sleep (93; 108). This discovery made narcolepsy the first identified primary sleep disorder; that is, a disorder associated with abnormalities of the sleep process related to brain dysfunction. The discovery of the hypocretin-1 deficiency syndrome responsible for narcolepsy certified this concept.
In the mid-1960s, 2 other discoveries led to the recognition that sleep could facilitate the appearance of specific disorders. The first was the identification of abnormal breathing patterns during sleep in association with obesity (Pickwickian syndrome) (39; 61); the second was the discovery of regular recurring patterns of leg movements during sleep (70), now called periodic limb movements of sleep. Further study of breathing during sleep led to the recognition of the importance of upper airway occlusion as a primary cause of sleep-related breathing disturbance (95).
Sleep disturbances have taken central stage in cognitive degradation. A variety of disorders such as REM sleep behavior disorder (RBD), excessive daytime sleepiness, and sleep apnea are emerging as strong vascular risk factors affecting the brain.
In the 1970s, the first sleep clinics devoted specifically to diagnosis and treatment of a broad range of sleep disorders were created. The Association of Sleep Disorders Centers, organized in 1975, provided a focus for development of the field of sleep disorders medicine. Subsequent major events included the publication of Sleep, the first journal devoted specifically to sleep disorders medicine. As the breadth of the field emerged, it became apparent that a nosology devoted to sleep disorders was required. The Association of Sleep Disorders Centers published the first classification of sleep disorders in 1979 (09). A more comprehensive classification was published by the American Sleep Disorders Association in 1990 as the International Classification of Sleep Disorders, and revised in 2000 (04). It was updated in 2005 by the American Academy of Sleep Medicine and again in 2014 (03) (Table 1). The American Board of Sleep Medicine (ABSM) was established in 1979 with the objective to promote excellence in medical care for patients with sleep disorders. ABSM certification is a lifetime credential that was intended to provide a standard of excellence by which “the public and members of government and regulatory bodies could identify skill, experience and competence”. Starting in 2007, authorized member boards of the American Board of Medical Specialties (American Board of Psychiatry and Neurology, American Board of Family Medicine, American Board of Internal Medicine, American Board of Otolaryngology and American Board of Pediatrics) began offering certification exams in sleep medicine every 2 years. Waivers of formal fellowship training to apply for the new exam expired in 2011. Diplomates are required to recertify their title every 10 years by complying with Maintenance of Certification normatives, some of which have been contested by specialty associations (109).
Table 1. International Classification of Sleep Disorders
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2. Sleep-related breathing disorders
3. Central disorders of hypersomnolence
4. Circadian rhythm sleep-wake disorders
6. Sleep-related movement disorders
7. Other sleep disorders
Appendix A: Sleep related medical and neurologic disorders
Appendix B: ICD-10-CM coding for substance-induced sleep disorders