During the last 20 years, the fact that the role of microarousal arousal is very involved in the pathophysiology of sleep disorders has been widely demonstrated. However, in spite of this, the nature of microarousal is still under debate. The awakening system forms a part of the relationship between activation system and sleep maintenance, thus providing the natural evolution of sleep and defending it from the stimuli of the external world. The awakening system, that includes the concept of microarousal, can appear isolated or periodically, then receiving the name of cyclic alternating pattern (CAP). CAP may be affected by different factors that can alter its periodicity, thus offering quantitative information on sleep instability. It can also serve as another new tool to understand human sleep and as complementary information supplied by the sleep macrostructure and microarousal study. This study aims to explain the concept of CAP and its pathophysiology studied up to the present.