Kcomplex

A K-complex is a distinctive waveform observed in the electroencephalogram (EEG) during non-rapid eye movement sleep, most commonly in stage N2. It appears as a single, sharp, high-amplitude wave with a negative polarity deflection that is quickly followed by a slower positive component. The waveform is typically maximal over frontocentral areas and lasts about 0.5 seconds, with peak-to-trough amplitudes often reaching 100 to 200 microvolts.

K-complexes can occur spontaneously during N2 sleep or be evoked by external stimuli, especially auditory stimuli such as a loud sound. They are often associated with a brief interruption in the ongoing EEG, and may be followed by a sleep spindle if arousal is mild. They can appear in clusters within a sleep period, reflecting fluctuations in arousal threshold.

Physiologically, K-complexes are thought to reflect transient thalamocortical–cortical interactions that temporarily suppress cortical arousal, thereby helping preserve sleep. They are considered to be one of the hallmark features of non-REM stage 2 sleep, alongside sleep spindles. Their precise functional role remains a topic of research; they are commonly described as a protective mechanism against awakening and may participate in memory-related processes during sleep.

In clinical practice, the presence of K-complexes helps identify N2 sleep in polysomnography. Their frequency and morphology can be influenced by factors such as age, sleep pressure, and certain sleep disorders, though a single K-complex is generally a normal finding in healthy sleepers. Abnormalities in K-complex patterns may be noted in some neurological or sleep-disordered states, but diagnosis typically relies on the overall pattern of sleep stages and other EEG features. See also sleep spindles, stage N2, and polysomnography.