Computer-generated cortical maps of power spectral estimates derived from 16 leads were drawn based on daytime sleep recordings in four normal volunteers. These data were compiled from nine 10-s artifact-free, EEG epochs from awake, stages 1-4 and REM sleep in each volunteer. EEG leads were placed on the left hemisphere and midline according to the 10-20 system with four additional interpolated posterior locations. Magnitude spectral estimates with 1 Hz resolution and adjacent frequencies (delta 2-4, alpha 8-12, beta 13-18) were analyzed with two-way ANOVA (lead by sleep stage). Delta activity was relatively uniform and of low amplitude in awake, eyes-closed subjects, and REM. Delta power increased at the vertex in stage 1. With progressing, non-REM sleep stages, it increased in power and enlarged radially to the intraparietal sulcus posteriorly, and the superior frontal gyrus anteriorly. Comparison of maps with ear and a computed average reference yielded similar topographic patterns. Alpha activity was expectedly maximal occipitally in awake subjects, but surprisingly a frontal area appeared in slow wave sleep. Beta activity in awake subjects was low and maximal parietally; stages 1 and REM showed even lower and more uniform distribution. Stage 2 showed the greatest power, concentrated at the vertex, with stages 3 and 4 diminishing. These data suggest that sleep stages are not completely uniform electrophysiologically across the cortex. This opens the possibility for a new method for the diagnosis of sleep disorders and alternatives in sleep staging.