<P> Some researchers argue that the perpetuation of a complex brain process such as REM sleep indicates that it serves an important function for the survival of mammalian and avian species . It fulfills important physiological needs vital for survival to the extent that prolonged REM sleep deprivation leads to death in experimental animals . In both humans and experimental animals, REM sleep loss leads to several behavioral and physiological abnormalities . Loss of REM sleep has been noticed during various natural and experimental infections . Survivability of the experimental animals decreases when REM sleep is totally attenuated during infection; this leads to the possibility that the quality and quantity of REM sleep is generally essential for normal body physiology . Further, the existence of a "REM rebound" effect suggests the possibility of a biological need for REM sleep . </P> <P> While the precise function of REM sleep is not well understood, several theories have been proposed . </P> <P> Sleep in general aids memory . REM sleep may favor the preservation of certain types of memories: specifically, procedural memory, spatial memory, and emotional memory . In rats, REM sleep increases following intensive learning, especially several hours after, and sometimes for multiple nights . Experimental REM sleep deprivation has sometimes inhibited memory consolidation, especially regarding complex processes (e.g., how to escape from an elaborate maze). In humans, the best evidence for REM's improvement of memory pertains to learning of procedures--new ways of moving the body (such as trampoline jumping), and new techniques of problem solving . REM deprivation seemed to impair declarative (i.e., factual) memory only in more complex cases, such as memories of longer stories . REM sleep apparently counteracts attempts to suppress certain thoughts . </P> <P> According to the dual - process hypothesis of sleep and memory, the two major phases of sleep correspond to different types of memory . "Night half" studies have tested this hypothesis with memory tasks either begun before sleep and assessed in the middle of the night, or begun in the middle of the night and assessed in the morning . Slow - wave sleep, part of non-REM sleep, appears to be important for declarative memory . Artificial enhancement of the non-REM sleep improves the next - day recall of memorized pairs of words . Tucker et al. demonstrated that a daytime nap containing solely non-REM sleep enhances declarative memory but not procedural memory . According to the sequential hypothesis the two types of sleep work together to consolidate memory . </P>

During rem sleep the muscles of the body core