Typical onset of narcolepsy occurs between the ages of ten and twenty; but can also occur in young children. It affects 1 in every 2,000 individuals who have a genetic pre-disposition that is then activated by an environmental trigger. In individuals with narcolepsy, aspects of rapid eye movement (REM) sleep intrude on wakefulness, while wakefulness intrudes on their sleep. Individuals with narcolepsy may quickly enter into REM sleep without first experiencing non-rapid eye movement (NREM) sleep, both at night and during the day. Some of the characteristics of narcolepsy — such as cataplexy, sleep paralysis and hallucinations — are similar to changes that occur in REM sleep, but instead, they occur during wakefulness or drowsiness. There are two main types of narcolepsy: narcolepsy with cataplexy (Type 1) and narcolepsy without cataplexy (Type 2). Symptoms vary from individual to individual, with some cases being more severe than others.