These disturbances tend to begin subtly and may change dramatically over time. The most common major symptom, other than excessive daytime sleepiness, is cataplexy, which occurs in about 70 percent of all people with narcolepsy. Sleep paralysis and hallucinations can also occur, but are somewhat less common. Only 10 to 25 percent of affected individuals, however, display all four of these major symptoms during the course of their illness. Narcolepsy may have several causes. Most people with narcolepsy have low levels of the neurotransmitter hypocretin, which promotes wakefulness. Most cases of narcolepsy occur in individuals with no known family history of the disorder. That being said, clusters in families can sometimes occur. Up to 10 percent of individuals diagnosed with narcolepsy with cataplexy report having a close relative with the same symptoms. While close relatives of people with narcolepsy have a statistically higher risk of developing the disorder than do members of the general population, that risk remains low when compared to diseases that are purely genetic in origin. In extremely rare cases, narcolepsy is caused by a genetic defect that prevents normal production of hypocretin
These disturbances tend to begin subtly and may change dramatically over time. The most common major symptom, other than excessive daytime sleepiness, is cataplexy, which occurs in about 70 percent of all people with narcolepsy. Sleep paralysis and hallucinations can also occur, but are somewhat less common. Only 10 to 25 percent of affected individuals, however, display all four of these major symptoms during the course of their illness. Narcolepsy may have several causes. Most people with narcolepsy have low levels of the neurotransmitter hypocretin, which promotes wakefulness. Most cases of narcolepsy occur in individuals with no known family history of the disorder. That being said, clusters in families can sometimes occur. Up to 10 percent of individuals diagnosed with narcolepsy with cataplexy report having a close relative with the same symptoms. While close relatives of people with narcolepsy have a statistically higher risk of developing the disorder than do members of the general population, that risk remains low when compared to diseases that are purely genetic in origin. In extremely rare cases, narcolepsy is caused by a genetic defect that prevents normal production of hypocretin