[Kanashibari Phenomenon as a Sleep Disorder: Recurrent Idiopathic Sleep Paralysis]

The phenomenon called kanashibari in Japan is otherwise known as sleep paralysis and, is an established symptom of narcolepsy. Its physiological correlate is an unusual occurrence of rapid eye movement (REM) sleep, i.e., sleep onset REM periods (SOREMPs). Various symptoms of the phenomenon are explained by the mechanisms of REM sleep. SOREMPs can be observed in various situations, which indicate a lower amplitude of circadian rhythms, such as during interupted sleep, acute reversal of sleep and wakefulness cycle, and disrupted sleep of depressive patients and in newborn babies. The lifetime prevalence of the phenomenon in the general population is as high about 40%. The phenomenon can be a mysterious and frightening experience far affected people and called as various names according to the folklore beliefs of local communities, e.g., nightmare in Europe, hexendrücken in Germany, cauchemar in France, and old hag in Newfoundland. Japanese college students are known for their very short sleep length and disturbed sleep wake rhythms. Many students claim to have experienced daytime sleepiness. Some of them meet the criterion concerning sleep latencies and frequency of SOREMPs for multiple sleep latency test (MSLT) as narcolepsy. SOREMPs are frequently seen in healthy college students with certain lifestyles. Practitioners should be cautious to diagnose narcolepsy if the student shows disturbed sleep wake schedules.