Rapid eye movement (REM) is a cyclical sleep period, occurring in intervals of 90-120 minutes during the night, occupying approximately 20-25% of total sleep time. During REM sleep, there is active inhibition of muscle activity, leading to complete or near complete atonia. This is a complex process generated by the interplay of multiple neurotransmitter systems, with a decrease in excitatory activity and increase in the inhibitory glycinergic and GABAergic premotor neuronal input to motor neurons. This book discusses the characteristics, disorders and physiological effects of REM sleep.