Interaction between prenatal hypoxia and the circadian system. Prenatal hypoxia decreases oxygen bioavailability in the tissues of the foetus. The sensitivity of the cell to decreased oxygen supply is mediated by hypoxia-inducible transcription factors, which interact with multiple physiological systems, including bi-directional interaction with the circadian system. Transcription factors induced by hypoxia show daily rhythms and directly bind to the E-box of the clock genes, thereby modulating the function of the circadian system. Transcriptional factors that respond to hypoxia also interact with the cardiovascular regulatory mechanisms to maintain homeostasis and sufficient blood flow through vital organs by increased peripheral resistance and reduction of oxygen consumption. ANS, autonomic nervous system; NO, nitric oxide; ROS, reactive oxygen species; RAAS, renin-angiotensin-aldosterone system.