K complex as an elementary form of arousal was investigated by evoked K complexes. The components building up evoked K complexes and topographical sleep-level differences and influence of the kind of cognitive elaboration were analysed. Evidences are presented supporting that K complex represents a cascade of events originating from sources of different topography and probably by a different kind of elaboration activated in a certain order requested by the nature and context of the eliciting stimuli. The power spectra of evoked arousals--including K complexes--were investigated. The poststimulus spectral pattern is characterized by a short initial power elevation and a following reduction of all frequency bands except a simultaneous but prolonged (5-20 s) and strong (50%) power reduction at the 13-14 Hz sigma spindle band. This phenomenon seems to be a common feature in different stages of slow wave sleep. This stimulus-related microstate could serve as a transitory stand-by state ready to reach higher arousal rapidly while maintaining the continuity of sleep; hence, the inhibition of spindle activity could provide a phasically improved thalamocortical sensory inflow after environmental stimuli. A microstructural scheme of arousals without awakening has been delineated.