<P> Rapid eye movement sleep (REM sleep, 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 . </P> <P> 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 . </P> <P> 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 . </P>

Rem sleep is also called paradoxical sleep because