<p>Periodic hypoxia consists of periods of low oxygen content alternating with periods of reoxygenation.Periodic hypoxia is usually associated with pathological conditions and is regarded as a negative stimulusleading to impairment of cognitive functions, the cardiovascular system, respiration, and metabolism.Periodic hypoxia is the most important pathophysiological element of sleep apnea syndrome, which isaccompanied by the development of inflammation and oxidative stress. On the other hand, periodic hypoxianot associated with sleep apnea has positive or negative effects on the body, depending on the intensity,duration, number, and frequency of exposures to hypoxia. In medicine, controlled interval hypoxia is usedto improve health and performance and is a non-drug treatment for a number of diseases. The objective ofthis review is to summarize clinical and experimental data on the effects of periodic hypoxemic hypoxiaon the adult body. Particular attention is paid to the effects of periodic hypoxia on cognitive functions. Thisreview discusses the molecular mechanisms by which periodic hypoxia can trigger a cascade of intracellularreactions leading to either beneficial or pathological effects.</p>