The biology of rapid eye movement sleep has puzzled scientists since its initial description in 1953. Challenging existing claims concerning the functions of Rapid Eye Movement sleep and the purported meaninglessness of dreams, this text offers a complete and up-to-date survey on the anatomy, physiology, ontogeny, and phylogeny of REM sleep as well as the cognitive neuroscience of dream phenomonolgy and dream content. The text underlines the importance of looking at how REM interacts physiologically with NREM sleep, in order to understand the potential functions of REM. The findings support and extend clams that the functions of REM involve memory consolidation and regulation of emotional conflicts and expression.