The study of dreams is called oneirology. Dreams occur during the rapid eye movement or REM stage of sleep. During this stage, a person appears to be awake due to continuous eye movements. At this stage dreams occur and can be seconds or as long as several minutes. Dreams can be real, surreal, magical, scary, sexual, and others. They contain images, emotions, thoughts and sensations. No one is particularly sure what the purpose of dreaming is. Freud believed that dreams were the result of our fears, desires etc and tried to interpret their meaning. Hobsen and McCarley's theory is that the brain stem is the source of signals interpreted as dreams working during REM sleep. Jie Zhang proposed a theory that dreams are the result of brain activation and synthesis. In other words, during REM the unconscious part of the brain is processing memory and this would flow to the conscious part of the brain. However, little evidence supports this theory. Some speculate that dreams contain hidden truths. Many cultures and groups have tried to decode dreams but this is still an area with much speculation and varying theories.