EVER SINCE Aserinsky's and Kleit-man's epoch-making discovery1 dreams have come to be inextricably linked to emergent EEG stage 1 rapid eye movement (REM) sleep.2-6 Other evidence, however, has indicated that mental activity goes on during all stages of sleep7,8: that what distinguishes REM from non-REM (NREM) sleep is not the presence but the qualitative uniqueness of the mental content associated with REM sleep. According to Foulkes7 and Rechtschaffen et al,9 REM mentation is more dream-like, more elaborate and complex, more vivid, and more visual than NREM mentation, while NREM mentation tends to be more thought-like, rational, conceptual, and realistic. It is understandable therefore why so much recent interest has focused on distinguishing different sleep stages not only on the basis of neurophysiological and biochemical processes, but also on the basis of the types of thinking that characterize them.
HARRY FISS, GEORGE S. KLEIN, EDWIN BOKERT. Waking Fantasies Following Interruption of Two Types of Sleep. Arch Gen Psychiatry. 1966;14(5):543–551. doi:10.1001/archpsyc.1966.01730110095015