Indications of mental activity are gathered from subjects during all four

stages of the sleep cycle as described by Kleitman and his associates. Whereas previous investigations have tended to assign dreaming to stage I (characterized in terms of EEG activity by the complete lack of spindles and by low-voltage, nonperiodic activity with occasional bursts of alpha rhythm, and by rapid eye movements), this study finds evidence of dreaming in the other stages of the sleep cycle as well. However, dreams during these other phases possess certain differences in formal characteristics from those of stage I. Such differences concern a heightened affective involvement, greater use of visualization and of physical movement, increased distortion, and greater distance from external reality in the dreams occurring during stage I of the sleep cycle.