K-complexes are very high amplitude, biphasic waveforms. They have an initial negative phase, then a large slow positive phase; usually, this large wave is followed immediately by a sleep spindle. N2 sleep is marked by sleep spindles and K-complexes. K-complexes start to appear in N2 sleep and can continue in N3 sleep. Particularly in younger patients, K-complexes can be extremely high amplitude (they are sometimes called the highest amplitude normal activity the brain produces).
K-complexes occur spontaneously or can be induced by auditory stimuli such as a click sound or noise. They are named K-complexes because in early sleep studies researchers noted that knocks or other noises while a patient was asleep brought them on. K-complex is considered a part of an arousal response and is often followed by an arousal pattern with diffuse alpha or theta activity.
