This article reviews the 15 years of studies conducted on the cyclic alternating pattern (CAP), the electro- encephalogram (EEG) marker of arousal instability during sleep. Starting from the limits and drawbacks of conventional sleep parameters (macrostructure), investigation of the underground world (microstructure) is extensively carried out with particular attention to K-complexes and other arousal-related phasic events of non-rapid eye movement (NREM) sleep. The continuity between conventional and non-conventional arousals is stressed in the light of autonomic function and motor activity evidence. The similarities and differences between CAP and arousals are thoroughly illustrated as well as the putative biological bases of these phenomena. The oscillatory nature of CAP, the involvement of CAP in the modulation of EEG synchrony, the sleep-maintenance properties of CAP in response to arousing stimuli, the clinical applications of CAP parameters and the promising availability of automatic scoring systems are highlighted.