In which stage of sleep are K-complexes predominantly observed?

K-complexes are predominantly observed during Stage 2 of sleep, which is characterized by a mix of sleep spindles and K-complexes, contributing to the overall architecture of this sleep stage. K-complexes are sudden, large waves that can occur in response to external stimuli, and they play a role in suppressing arousal in order to maintain sleep. This phenomenon is significant because it helps the individual remain asleep despite potential disturbances, enhancing the restorative functions of sleep.

In this stage, the brain exhibits distinct patterns that signify a deeper state of sleep compared to Stage 1, where light sleep occurs, and K-complexes are not prevalent. Stages 3 and REM sleep showcase different brain activities; Stage 3 comprises slow-wave sleep with delta waves, while REM sleep is marked by rapid eye movements and a more active brain state similar to wakefulness, where K-complexes are generally absent. Thus, Stage 2 is the key phase for the appearance of K-complexes, making it the correct answer to the question.