A K-complex is an EEG waveform that occurs during stage 2 sleep. It consists of a brief high-voltage peak, usually greater than 100 µV, and lasts for longer than 0.5 seconds. K-complexes are often followed by bursts of sleep spindles. K-complexes occur randomly throughout stage 2 sleep, but may also occur in response to auditory stimuli. It is the response to auditory stimuli that likely lead to the origin of the term K-complex. Original research in the mid-1900s showed that K-complexes would arise when researchers would knock on the door of a sleeping subject.