They occur spontaneously but also occur in response to external stimuli such as sounds, and touches on the skin and internal ones such as inspiratory interruptions. They are generated in widespread cortical locations though they tend to predominate over the frontal parts of the brain, 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.