The electroencephalograms (EEGs) of the sleep onset period (SOP) of psychophysiogical insomniacs. psychiatric insomniacs, and controls were compared using power spectral analysis. We predicted that psychophysiological insomniacs would show elevated cortical arousal throughout their entry into sleep. Electroencephalograms, electrooculograms (EOGs), and electromyograms (EMGs) were recorded for two consecutive nights. Power spectral analysis of EEG from the sleep onset period was performed on all standard frequency bands. Psychophysiological insomniacs had less alpha during the first part of the SOP and did not show the dramatic drop in alpha across the SOP that characterizes normal sleep. They showed less delta in the last quartile of the chronological analysis of the SOP. Psychiatric insomniacs showed lower relative beta power values overall, while psychophysiological insomniacs showed higher relative beta power values during wakefulness. This microanalysis indicates that the SOP is generally similar for psychiatric insomniacs and normal sleepers but that clear differences in the SOP of psychophysiological insomniacs exist. They have higher cortical arousal during the SOP than do psychiatric insomniacs and controls. The dramatic changes in power values in the latter two groups as sleep begins are not seen in the psychophysiological insomniacs, which may help explain the difficulty that psychophysiological insomniacs have discriminating between wakefulness and sleep.