The Cyclic Alternating Pattern and the Brain-Body-Coupling During Sleep

Sleep measurement is based on conventional rules, which simplify the complex architecture of this mandatory phsyiological activity. The dynamic development of sleep shows a regular cyclic nature, which is reflected also in the microstructural organization of phasic EEG events. In NREM sleep, arousals rarely appear in isolation but are most commonly arranged in sequences recognized as the cyclic alternating pattern (CAP). Composed of EEG features endowed with properties of activation (phase A) and deactivation (phase B), CAP translates a condition of sustained arousal instability. Due to the close temporal relation between CAP, autonomic functions and behavioral activities, CAP operates as a master clock that entrains different rhythms in a common temporal pattern. Crucially participating in the build-up, maintenance and attenuation of slow wave sleep, CAP represents a fundamental pillar of sleep processes together with the homeostatic drive, the ultradian cyclicity and the circadian oscillation. The pivotal role of CAP in the basic sleep mechanisms justifies its involvement in the pathophysiology of most sleep disorders and offers new perspectives in clinical management strategies. The absence of CAP (defined as nonCAP) corresponds to a sustained condition of stability extended to both brain and body. CAP and non-CAP metrics overcome the rigid limitations of conventional stage scoring and provide a more flexible and genuine neurophysiological substrate to shed light upon the brain-body coupling during sleep.