Hypnagogic imagery—the fleeting visual or sensory experiences that occur at the onset of sleep—varies systematically with specific brainwave stages. Seven adults reported their imagery each minute while their EEG activity was classified into five stages using Hori's criteria. The content of the imagery shifted predictably as the EEG progressed through these stages, indicating a close link between subjective experience and measurable brain states during the transition to sleep.
Hypnagogic EEG stages, defined by Hori's nine-stage system, show distinct topographic patterns of brain activity as sleep progresses. Delta and theta power increase prominently in anterior-central areas starting from the vertex sharp-wave stage (stages 6 and 7). Alpha 3 power increases in anterior-central areas from stage 9, and sigma power increases in the central-parietal area from stage 8. These findings suggest that basic sleep processes may begin before the onset of sleep stage 2 or before manually scored sleep spindles appear.