DREAMS AND THE SLEEP CYCLE
In their earliest report, Aserinsky and Kleitman1 found that 74% of awakenings from REM sleep resulted in recall of a dream, as compared with only 9% of awakenings from NREM sleep. The association between dreaming and REM sleep was subsequently replicated by many other investigators; typically, around 80% of REM awakenings yield dreams. The much greater likelihood
of obtaining a dream report from REM sleep led some early researchers to conclude that all dreaming, and even all mental activity, during sleep occurred in REM sleep, with reports from NREM sleep reflecting recall of mental activity from an earlier REM period, or mentation
arising during the arousal itself. On the other hand, several laboratories reported much higher frequencies of NREM reports than in the earliest studies.3 Some of the differences reflected less stringent criteria for what constitutes a dream. It became clear over time that there is a
good deal of mental activity that occurs during NREM sleep. Typically, it is more thoughtlike, fragmentary, and related to daily concerns than the vivid, hallucinatory, predominantly visual narratives that are most commonly reported from REM sleep. But even this distinction appears not to be absolute. There is now wide acceptance of the view that some dreaming that is indistinguishable from REM sleep dreaming occurs in NREM sleep, most frequently in the sleep-onset period.