There has been a significant increase during the last decades in knowledge of genetics of sleep and sleep disorders, and the genetic epidemiologic studies have considerably contributed to this progress in understanding their basis. The primary goal of genetic epidemiology is the resolution of the genetic architecture of a trait, such as sleep length or a disorder. Electroencephalogram (EEG), a parameter included in polysomnography (PSG), has been found to be one of the most heritable characteristics, with heritability estimates greater than 95%, in a sample of 10 MZ and 10 DZ twin pairs. Most studies indicate that certain sleep problems in childhood are largely influenced by genes. Most parasomnias are relatively common to very common in childhood, occurring clearly less frequently in adults. Clinical experience and many studies indicate that parasomnias are often found to co-occur and run in families.