The rhythmicity of life: A review of the circadian clocks

Physiology of the mammalian body has been adapted to diurnal cycles of around 24 h, an evolutionary situation that affects a wide spectrum of biological events including sleep-to-wake transitions, feeding/fasting, body temperature, and hormonal regulations. The patterns of the diurnal cycle occur due to rhythmic oscillations that arise from the suprachiasmatic nucleus of hypothalamus, which also can be defined as the pacemaker of the system. The clock can be defined as a molecular machinery driven by the core clock genes that encode clock proteins in a rhythmic oscillatory fashion maintained by the light/dark cycles of the environment. Although the well-established knowledge refers to the function of the circadian rhythm as maintenance of the normal physiology, growing evidence shows that disruptions in the system usually caused by genetic and/or epigenetic misregulations may have a direct effect to lead major pathological conditions, such as carcinogenesis. This review outlines the main molecular aspects of circadian physiology, and reveals the reasons for and results of the circadian disruptions at different levels. In spite of the fact that more proof is needed for a direct correlation between circadian disruptions and oncogenesis and other pathological events, data obtained from current research supports the role of circadian rhythms in malfunctioning of the normal cellular metabolism.