Rapid Eye Movements
Rapid eye movement sleep is a unique phase of sleep in mammals and birds, characterized by random rapid movement of the eyes, accompanied by low muscle tone throughout the body, and the propensity of the sleeper to dream vividly. The REM phase is also known as paradoxical sleep 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. REM and non-REM sleep alternate within one sleep cycle, which lasts about 90 minutes in adult humans.
