The genetics of sleep and sleep disorders is still largely unknown and not well understood; however, new studies show the importance not only for understanding brain physiology but for sleep disorders and the circadian regulation that influences most body systems. In order to understand the physiology and pathophysiology of sleep, genetic studies are being developed that include new genetic techniques to tell us not only about brain regions that are activated or deactivated by sleep and alertness but also help us understand the pathophysiological mechanisms involved. This book, Genetics of Sleep and Sleep Disorders, details the important advances in the genetics of sleep disorders that hold promise to help us understand the underlying physiology and pathophysiology of sleep that will also aid in the diagnosis of sleep disorders. There has been a major increase during the last decades in knowledge of the genetics of sleep and sleep disorders. Genetic epidemiologic studies have contributed considerably; however, there are marked differences in the level of knowledge between different aspects of sleep and individual disorders. Linkage, genome-wide association, and sequencing are yielding new insights into the basis of sleep traits. Mutations in the clock genes have been associated with Mendelian alterations of circadian rhythms and candidate gene association studies have been reported for a variety of sleep disorders. Most sleep disorders are considered to be complex genetic disorders. Recent progress has been made in identifying the genetic basis of narcolepsy and RLS and genome- wide association studies have demonstrated several genetic loci associated with their pathogenesis. The genetic basis remains to be determined for the more prevalent sleep disorders, insomnia and obstructive sleep apnea. Epigenetic mechanisms are being recognized as playing a major part in gene regulation of sleep. In the future whole-genome sequencing may clarify the genetic basis of complex traits including those associated with circadian sleep–wake regulation and help discover new gene networks involved in the regulation of sleep and the pathogenesis of sleep disorders.