K-complex (KC) is a major component of electroencephalography (EEG) in human brain during non-rapid eye movement sleep, and there has long been a debate on its function. Some researchers consider that KC represents a sign of arousal response of brain to internal or external stimuli, and others suggest that it is a protective response to sustain sleep. In this article we put forward a functional model of KC based on the above two views and related literatures. According to our model, KC arises from three kinds of brain rhythms (sleep cyclic, central oscillation, and cortical oscillation). Based on the results of stimulus significance assessed by sleep-information assessment center, the generation of KC is related to three different responses-total arousal, sign of being awakened, and no sign of being awakened. In conclusion, KC is a reflection of cerebral state responsive to stimuli.