Normal sleep is made up of two phases, first one ir non-rapid eye movement (NREM) sleep, and the second one is rapid eye movement (REM) sleep. This two alternate and that makes a cicle; each cycle consists of NREM sleep followed by REM sleep. REM sleep start at 90 minutoes of sleep.
REM slep have chemical and electrical activities that regulate it, during this phace the brain have abundance of the neurotransmitter acetylcholine, combined with lack of monoamine neurotransmitters histamine, serotonin and norepinephrine. Some studies theorize that this make REM sleep involved in the functions of memory, however, there is not enough information to ensure that memory depends on REM sleep.
