Rapid eye movement sleep (
REM sleep,
REMS) is a unique phase of mammalian
sleep characterized by random movement of the
eyes, low
muscle tone throughout the body, and the propensity of the sleeper to
dream vividly. This 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 norepinepherine. The cortical and thalamic neurons of the waking or paradoxically sleeping brain are more depolarized—i.e., can "fire" more readily—than in the deeply sleeping brain. The right and left hemispheres of the brain are more
coherent in REM sleep, especially during
lucid dreams.