The relevance of sleep instability is poorly appreciated among the metrics of sleep physiology. The cyclic alternating pattern (CAP) is a periodic electroencephalogram activity of non-REM sleep, characterized by sequences of transient electrocortical events that are distinct from the tonic background and recur at up to 1-min intervals. In the dynamic organization of sleep, CAP expresses a condition of instability that reflects the brain's effort in preserving and regulating the physiological structure of sleep.