Circadian Clocks in Mammals: General Aspects of Oscillating Systems and Roles in Medicine

Abstract

The circadian system generates about 24-hour rhythms in body functions to adjust their performances to cyclic environments, thereby ensuring survival. Misalignment of internal periodicities and environmental cycles eventually leads to both mental and physical malfunctions. To gain insight into misalignment, it is absolutely necessary to understand the principle of entrainment, especially in relation to the light–dark (LD) cycle. Non-parametric and parametric entrainments are basic concepts and two coupled oscillators are likely involved in stable entrainment and its seasonal changes. It is critical to understand the molecular mechanism of the circadian system to comprehend the regulation of the system. The discovery of clock genes has greatly increased our knowledge but we are now grappling with the complexity of the system. The suprachiasmatic nucleus in the hypothalamus serves as the central clock, which entrains to the LD cycle and regulates the peripheral clocks located in almost all tissues and organs. Modern society is chaotic in terms of time structure, where the circadian system is easily disrupted. Understanding the mechanism underlying circadian disorders is important for their prevention and treatment. The concept of chronomedicine, which was proposed several decades ago, is now ready to be materialized with the advent of molecular chronobiology.