The present study deals with one of the fundamental psychoanalytic assumptions: the existence of the primary and secondary process. It can be hypothesized that these two different ways of mental functioning differentiate not only conscious and unconscious processes but also REM- and non-REM-sleep. In REM-sleep, when most bizarre dreams occur, the primary process is supposed to be the dominant mode of thinking while non-REM-sleep, when dreams are mainly characterized as thought-like, is supposed to be organized along secondary process lines. So far, only one single study tried to test this hypothesis applying the method of subliminal stimulation on the sleep-dream cycle (Shevrin & Fisher, 1967). In this experimental study we successfully replicated these early findings. Moreover, additional stimuli were used and neurophysiological measurements recorded. Our findings demonstrate that REM- and non-REM-sleep can indeed be differentiated on the basis of their prevailing kind of mental functioning matching the Freudian idea of primary and secondary process thinking
