Definition reprinted with permission from Culebras, Antonio, Clinical Handbook of Sleep Disorders, Butterworth-Heinemann
A sharp, negative EEG wave followed by a high-voltage slow wave. The complex duration is at least 0.5 seconds and may be accompanied by a sleep spindle. K complexes occur spontaneously during non-REM sleep, and begin and define stage 2 sleep. They are thought to be evoked responses to internal stimuli. They can also be elicited during sleep by external (particularly auditory) stimuli.