REM stands for Rapid Eye Movement. REM sleep is characterized by the sleeper’s eyes moving rapidly beneath closed eyelids. The length of time spent in REM sleep increases throughout the night with each progressive sleep cycle, so most of it is experienced closer to morning. Brain activity is very high during REM sleep. During REM sleep, most muscles are paralysed.
REM sleep is considered to be the most mentally restorative sleep, and is the stage that produces the most memorable dreams, which is when the brain is developing. REM sleep is considered to be critical for the consolidation of certain types of memory (spatial and procedural), and is also strongly associated with problem solving and creativity.