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K complex

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.

Definition reprinted with permission from Culebras, Antonio, Clinical Handbook of Sleep Disorders, Butterworth-Heinemann