We typically spend more than two hours each night dreaming. Scientists do not know much about how or why we dream. Sigmund Freud, a major influence in psychology, believed dreaming was a "safety valve" for unconscious desires. Only after 1953, when researchers first described REM in sleeping infants, did scientists begin studying sleeping and dreaming carefully. They soon discovered that the strange, illogical experiences we call dreams almost always occur during the REM part of sleep. While most mammals and birds show signs of REM sleep, reptiles and other cold-blooded animals do not.
REM sleep begins with signals from the pons, an area at the base of the brain. These signals travel to a region called the thalamus, which relays them to the cerebral cortex-the outer layer of the brain responsible for learning, thinking and organizing information. The pons sends signals that shut off neurons in the spinal cord, causing temporary paralysis of limb muscles. If something interferes with this paralysis, people will begin to physically act out their dreams-a rare, dangerous problem called REM sleep behavior disorder. For example, a person dreaming about a baseball game may run into furniture or inadvertently strike a person sleeping nearby while trying to catch a ball in the dream.
REM sleep stimulates the brain regions used in learning, which may be important for normal brain development during infancy. This would explain why infants spend much more time in REM sleep than adults. Like deep sleep, REM sleep is associated with increased production of proteins. One study determined that REM sleep affects learning certain mental skills. People taught a skill and then deprived of non-REM sleep were able to recall what they had learned after sleeping, while people deprived of REM sleep were not.
Some scientists believe dreams are the cortex's attempt to find meaning in the random signals received during REM sleep. The cortex is the part of the brain that interprets and organizes information from the environment during consciousness. One theory suggests that, given random signals from the pons during REM sleep, the cortex, attempting to interpret these signals, creates a "story" out of fragmented brain activity
