<P> After waking from REM sleep, the mind seems "hyperassociative"--more receptive to semantic priming effects . People awakened from REM have performed better on tasks like anagrams and creative problem solving . </P> <P> Sleep aids the process by which creativity forms associative elements into new combinations that are useful or meet some requirement . This occurs in REM sleep rather than in NREM sleep . Rather than being due to memory processes, this has been attributed to changes during REM sleep in cholinergic and noradrenergic neuromodulation . High levels of acetylcholine in the hippocampus suppress feedback from hippocampus to the neocortex, while lower levels of acetylcholine and norepinephrine in the neocortex encourage the uncontrolled spread of associational activity within neocortical areas . This is in contrast to waking consciousness, where higher levels of norepinephrine and acetylcholine inhibit recurrent connections in the neocortex . REM sleep through this process adds creativity by allowing "neocortical structures to reorganise associative hierarchies, in which information from the hippocampus would be reinterpreted in relation to previous semantic representations or nodes ." </P> <P> In the ultradian sleep cycle an organism alternates between deep sleep (slow, large, synchronized brain waves) and paradoxical sleep (faster, desynchronized waves). Sleep happens in the context of the larger circadian rhythm, which influences sleepiness and physiological factors based on timekeepers within the body . Sleep can be distributed throughout the day or clustered during one part of the rhythm: in nocturnal animals, during the day, and in diurnal animals, at night . The organism returns to homeostatic regulation almost immediately after REM sleep ends . </P> <P> During a night of sleep, humans usually experience about four or five periods of REM sleep; they are shorter (~ 15m) at the beginning of the night and longer (~ 25m) toward the end . Many animals and some people tend to wake, or experience a period of very light sleep, for a short time immediately after a bout of REM . The relative amount of REM sleep varies considerably with age . A newborn baby spends more than 80% of total sleep time in REM . </P>

Besides dreaming what is rem sleep characterized by