V. Reports of Mental Activity during Sleep

The variety of mental activity during sleep was investigated by obtaining reports of this activity upon awakening from two different kinds of sleep: (1) REM (rapid eye-movement) periods, which are characterized by a low voltage, random EEG and the presence of rapid eye-movements, and (2) NREM periods, which are characterized by 12–14 cps sleep spindles and/or slow, high amplitude delta EEG activity and the absence of rapid eye-movements. A total of 186 awakening reports were obtained from 17 subjects who spent from one to three nights each sleeping in the laboratory. As had been described in earlier investigations, reports of mental activity during sleep were obtained more frequently on awakenings from REM periods than on awakenings from NREM periods. However, a considerable amount of mental activity was obtained on NREM awakenings. As compared with REM period mentation, this NREM mental activity was described as more like thinking and less like dreaming, less vivid, less visual, more conceptual, under greater volitional control, more plausible, more concerned with the contemporary life of the subject, occurring in lighter sleep, less emotional, and more pleasant. The NREM mental activity was interpreted as resembling the unfocused, floating, background mentation of waking life, and it was hypothesized that the poor recall for this type of mentation might be related to its relative lack of perceptual intensity.