RESUMO
The function of the circadian cycle is to determine the natural 24 h biological rhythm, which includes physiological, metabolic, and hormonal changes that occur daily in the body. This cycle is controlled by an internal biological clock that is present in the body's tissues and helps regulate various processes such as sleeping, eating, and others. Interestingly, animal models have provided enough evidence to assume that the alteration in the circadian system leads to the appearance of numerous diseases. Alterations in breathing patterns in lung diseases can modify oxygenation and the circadian cycles; however, the response mechanisms to hypoxia and their relationship with the clock genes are not fully understood. Hypoxia is a condition in which the lack of adequate oxygenation promotes adaptation mechanisms and is related to several genes that regulate the circadian cycles, the latter because hypoxia alters the production of melatonin and brain physiology. Additionally, the lack of oxygen alters the expression of clock genes, leading to an alteration in the regularity and precision of the circadian cycle. In this sense, hypoxia is a hallmark of a wide variety of lung diseases. In the present work, we intended to review the functional repercussions of hypoxia in the presence of asthma, chronic obstructive sleep apnea, lung cancer, idiopathic pulmonary fibrosis, obstructive sleep apnea, influenza, and COVID-19 and its repercussions on the circadian cycles.