Non-rapid eye movement sleep, or NREM, is, collectively, sleep stages 1–3, previously known as stages 1–4. Rapid eye movement sleep (REM) is not included. There are distinct electroencephalographic and other characteristics seen in each stage. Unlike REM sleep, there is usually little or no eye movement during these stages. Dreaming is rare during NREM sleep, and muscles are not paralyzed as in REM sleep. People who do not go through the sleeping stages properly get stuck in NREM sleep, and because muscles are not paralyzed a person may be able to sleepwalk. According to studies, the mental activity that takes place during NREM sleep is believed to be thought-like, whereas REM sleep includes hallucinatory and bizarre content. The mental activity that occurs in NREM and REM sleep is a result of two different generators, which also explains the difference in mental activity. In addition, there is a parasympathetic dominance during NREM. During the period of Non-REM sleep, the mindset of a person is more organized. The differences in the REM and NREM activity reported is believed to arise from differences in the memory stages that happen during the two methods of sleep. It has been found through several experiments that low levels of stage 3 sleep are found in about 40-50% of acute and chronic schizophrenics who typically portray abnormal non-rapid eye movement sleep.