If the notions of dream and nightmare are centuries old, going back to ancient Egyptian and Jewish civilizations, the distinction between nightmares and parasomnias is recent. As parasomnias became distinguishable from nightmares, a possible link between such episodic nocturnal phenomena and seizure disorders was proposed. In 1999, Ohayon et al. in their epidemiological studies on sleepwalking and sleep terrors found that obstructive sleep apnea syndrome was the most common sleep disorder associated with parasomnias between the ages of 15 and 24 years. Epileptic disorders were shown to be rarely involved in abnormal behavior during non-rapid eye movement (NREM) sleep, but when sleep-related seizure disorders are present, specific seizure entities are implicated. Nocturnal polysomnography has allowed the dissociation of NREM from REM sleep abnormal behavior. The initial description of what is now known as REM sleep behavior disorder (RBD) came from Japanese researchers.