"When the brain in not functioning properly during dreams of aggression, sleepwalkers have been able to commit murder. In several cases sleepwalkers did not recognize their victims. For example, one man drove 15 kilometers to his mother-in-law’s house, which means that his motor skills were intact. Then he stabbed her with a knife and did not respond to her screams. He acted as if he was under threat and initiated a fatal attack. This man had a genetic and personal history of awakening abruptly from the first cycle of sleep into a confused state and never entered REM sleep (Cartwright, 2000). The REM sleep is necessary to prevent waking hallucinations and mental illness (Siegel, 2001). Since aggression is prominent in most dreams, it is likely that our enemies cause this aggression. In a study by Hall and Van de Castle, animals and male strangers are the primary enemies in both male and female dreams. When a animal enters a dream it is almost always going to pose some threat or danger to the dreamer. The reason for this was thought to be because in ancestral times humans lived in an environment full of dangerous animals. There was also a constant threat from other humans. These ever-present dangers made behavioral strategies to avoid contact with these things and was of a high survival value. Dreaming simulates these strategies in order to maintain efficiency; otherwise, one failure to respond to these threats in waking life could mean death. Dreams are biased towards simulating threats that were common in our ancestral environment (Revonsuo, 2000)."