The five stages of the sleep-wake cycle are distinct with different patterns of brain wave activity (EEG), eye movements, and muscle tone (chin).

In Stage 4 SWS, the arousal threshold is very high. But if you’re awakened abruptly, you may experience “parasomnias” such as sleep walking, sleep talking, or night terrors (episodes of screaming and agitation during the first half of the night). All of these phenomena occur between sleep and wakefulness—you aren’t fully awake but you also aren’t fully asleep, and you usually transition back to sleep without waking up. Parasomnias are considered to be normal occurrences and happen more frequently in childhood and adolescence than in adulthood.