The relationship between the level of consciousness and the level of brain activation was studied. The model providing gradual decreases in the level of consciousness consisted of states in the transition from waking to drowsiness. The measure of the level of consciousness, measured from subjects’ self-assessments, was the change in the content of consciousness, i.e., the intensity and quality of involuntarily arising mental processes. Each of the levels of consciousness was found to correspond to a defined EEG profile. The results showed that the lowest level of activation corresponded to the state in which the subjects noted inhibition of “inner speech” and “cessation of thoughts,” this being associated with a change in the dynamics of the high-frequency EEG rhythm associated with the mechanism of transmission of cognitive information.