memories and reinforcing old memories. In this type of brain stimulation, the dreaming that occurs is a by-product of the process. Stages of sleep During normal sleep cycles, humans alternate between normal, NREM sleep and REM sleep. The brain waves characteristic of dreaming that are observed during REM sleep are the most commonly studied in dream research because most dreaming occurs during REM sleep. REM sleep In 1952, Eugene Aserinsky discovered REM sleep while working in the surgery of his PhD advisor. Aserinsky noticed that the sleepers' eyes fluttered beneath their closed eyelids, later using a polygraph machine to record their brain waves during these periods. In one session, he awakened a subject who was wailing and crying out during REM and confirmed his suspicion that dreaming was occurring. In 1953, Aserinsky and his advisor published the ground-breaking study in Science. Accumulated observation shows that dreams are strongly associated with REM sleep, during which an electroencephalogram shows brain activity to be most like wakefulness. Participant-nonremembered dreams during NREM are normally more mundane in comparison. During a typical lifespan, a human spends a total of about six years dreaming (which is about two hours each night). Most dreams last only 5 to 20 minutes. It is unknown where in the brain dreams originate, if there is a single origin for dreams, if multiple portions of the brain are involved, or what the purpose of dreaming is for the body or mind. During REM sleep, the release of certain neurotransmitters is completely suppressed. As a result, motor neurons are not stimulated, a condition known as REM atonia. This prevents dreams from resulting in dangerous movements of the body. Animals have complex dreams and are able to retain and recall long sequences of events while they are asleep. Studies show that various species of mammals and birds experience REM during sleep, and follow the same series of sleeping states as humans. The discovery that dreams take place primarily during a distinctive electrophysiological state of sleep (REM), which can be identified by objective criteria, led to rebirth of interest in this phenomenon. When REM sleep episodes were timed for their duration and subjects awakened to make reports before major editing or forgetting could take place, it was determined that subjects accurately matched the length of time they judged the dream narrative to occupy with the length of REM sleep that preceded the awakening. This