Rapid eye movement sleep (REM sleep or REMS) is a unique phase of sleep in mammals and birds, distinguishable by random/rapid movement of the eyes, accompanied with low muscle tone throughout the body, and the propensity of the sleeper to dream vividly. The REM phase is also known as paradoxical sleep (PS) and sometimes desynchronized sleep because of physiological similarities to waking states, including rapid, low-voltage desynchronized brain waves. Electrical and chemical activity regulating this phase seems to originate in the brain stem and is characterized most notably by an abundance of the neurotransmitter acetylcholine, combined with a nearly complete absence of monoamine neurotransmitters histamine, serotonin, and norepinephrine. REM sleep is physiologically different from the other phases of sleep, which are collectively referred to as non-REM sleep (NREM sleep, NREMS, synchronized sleep). REM and non-REM sleep alternate within one sleep cycle, which lasts about 90 minutes in adult humans. As sleep cycles continue, they shift towards a higher proportion of REM sleep. The transition to REM sleep brings marked physical changes, beginning with electrical bursts called PGO waves originating in the brain stem. Organisms in REM sleep suspend central homeostasis, allowing large fluctuations in respiration, thermoregulation, and circulation which do not occur in any other modes of sleeping or waking. The body abruptly loses muscle tone, a state known as REM atonia. Professor Nathaniel Kleitman and his student Eugene Aserinsky defined rapid eye movement and linked it to dreams in 1953. REM sleep was further described by researchers including William Dement and Michel Jouvet. Many experiments have involved awakening test subjects whenever they begin to enter the REM phase, thereby producing a state known as REM deprivation. Subjects allowed to sleep normally again usually experience a modest REM rebound. Techniques of neurosurgery, chemical injection, electroencephalography, positron emission tomography, and reports of dreamers upon waking, have all been used to study this phase of sleep.