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A K-complex is a waveform that may be seen on an electroencephalogram (EEG). It occurs during stage 2 NREM sleep. It is the "largest event in healthy human EEG". They are more frequent in the first sleep cycles.
K-complexes have two proposed functions:
Neurophysiology
K-complex consists of a brief negative high-voltage peak, usually greater than 100 μV, followed by a slower positive complex around 350 and 550 ms and at 900 ms a final negative peak. K-complexes occur roughly every 1.0–1.7 minutes and are often followed by bursts of sleep spindles. They occur spontaneously and internal ones such as inspiratory interruptions. They are generated in widespread cortical locations specifically the anterior and superior aspects of the medial and lateral frontal lobe cortices.
Both K-complex and delta wave activity in stage 2 sleep create slow-wave (0.8 Hz) and delta (1.6–4.0 Hz) oscillations. However, their topographical distribution is different, and the delta power of K-complexes is higher.
They are created by the occurrence in widespread cortical areas of outward dendritic currents from the middle (III) to the upper (I) layers of the cerebral cortex. This is accompanied by a decrease in broadband EEG power including gamma wave activity. This produces "down-states" of neuronal silence in which neural network activity is reduced. It has been observed that they are indeed identical in the "laminar distributions of transmembrane currents" to the slow waves of slow-wave sleep. This would be consistent with a function in suppressing cortical arousal in response to stimuli that the brain needs to initially process in regard to whether it is dangerous or not. and memory consolidation. The activation thresholds of cortical synapses become lowered during wakefulness as they process information, making them more responsive, and so need to be adjusted back to preserve their signal-to-noise ratio. This parallels the decrease in other components of sleep such as sleep spindle density and delta power. In autosomal dominant nocturnal frontal lobe epilepsy, K-complexes are almost invariably present at the start of seizures.
Restless legs syndrome
Individuals with restless legs syndrome have increased numbers of K-complexes, which are associated with (and often precede) leg movements. Dopamine enhancing drugs such as L-DOPA that reduce leg movements do not reduce the K-complex suggesting that they are primary and the leg movements secondary to them. Failure of such drugs to reduce K-complexes in spite of reducing the leg movements has been suggested to be why patients after such treatment continue to complain of non-restorative sleep.
Obstructive sleep apnea
Obstructive sleep apnea syndrome is associated with inspiratory occlusions evoking fewer K-complexes during NREM sleep even though K-complexes are evoked normally to auditory stimuli and such individuals react normally to respiratory interruptions when awake. This suggests a link between such sleep apnea and a sleep specific blunted cortical response to respiratory problems.