A characteristic high amplitude waveform that is observed in brain waves as recorded on electroencephalography (EEG) during stage 2 of non rapid-eye movement (NREM) sleep. It has a sharp negative deflection followed by a slower, positive component. The duration is defined as lasting at least a half second. Although they may occur spontaneously, K-complexes can also be elicited during sleep in response to auditory stimuli. It often occurs in stage 2 sleep with another waveform called a sleep spindle.