Relationships between known EEG changes occurring at sleep onset (SO) and processes of SO imagery formation are still poorly understood. In the present study, 24 healthy subjects signaled and reported spontaneous SO imagery while in a seated, head-unsupported position. Two judges rated the sensory content of all images. EEG samples immediately preceding the imagery signals as well as from preceding wakefulness were also recorded from a 19-channel montage. EEG samples were categorized by two judges into one of nine SO stages proposed by Hori et al. (1994). Unimodal kinesthetic images (apparent self-movement) and unimodal visual images accompanied only by SO-stage 4 were further subject to spectral analysis, topographically mapped and statistically compared. These SO images were characterized by significant decreases in all frequency bands except delta, for which significant increases were observed over several electrode sites. Kinesthetic and visual images were accompanied by different topographic patterns of delta power: kinesthetic, by prefrontal and frontal delta activation, and visual, by delta activation in more left-central and temporal regions. Results suggest that the documented spread of anterior to posterior delta power occurring for a brief window early in SO may be associated with sense-specific imagery processes unfolding over time. The results are also consistent with a novel explanation for the phenomenon of the "sleep start" which is commonly accompanied by vivid kinesthetic images of falling at the point of sleep onset.