Cyclic alternating pattern in polysomnography: what is it and what does it mean?

PURPOSE OF REVIEW: 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. RECENT FINDINGS: CAP quantification is a topical feature in the evaluation of sleep quality. In addition to duration, depth, and continuity, sleep restorative properties depend on the brain's capacity to determine the periods of sustained stable sleep. This issue is not confined only to the electroencephalogram activities but reverberates upon the ongoing autonomic and behavioral functions, which are mutually entrained in a synchronized oscillation. As a master clock involved in the dynamic organization of sleep, CAP plays a crucial role in numerous sleep disorders and is powerfully influenced by medication and appropriate treatment. SUMMARY: This article reviews the scoring, significance, and clinical applications of CAP.