International Journal of Molecular Zoology 2024, Vol.14, No.3, 166-181 http://animalscipublisher.com/index.php/ijmz 166 Feature Review Open Access Circadian Rhythms in Animals: Mechanisms and Functions Wei Liu, Jia Xuan Institute of Life Science, Jiyang College of Zhejiang A&F University, Zhuji, 311800, Zhejiang, China Corresponding author: jia.xuan@jicat.org International Journal of Molecular Zoology, 2024, Vol.14, No.3 doi: 10.5376/ijmz.2024.14.0016 Received: 01 Apr., 2024 Accepted: 11 May, 2024 Published: 01 Jun., 2024 Copyright © 2024 Liu and Xuan, This is an open access article published under the terms of the Creative Commons Attribution License, which permits unrestricted use, distribution, and reproduction in any medium, provided the original work is properly cited. Preferred citation for this article: Liu W., and Xuan J., 2024, Circadian rhythms in animals: mechanisms and functions, International Journal of Molecular Zoology, 14(3): 166-181 (doi: 10.5376/ijmz.2024.14.0016) Abstract Circadian rhythms are intrinsic, approximately 24-hour cycles that regulate various physiological and behavioral processes in animals. These rhythms are governed by molecular clocks composed of interlocked transcriptional-translational feedback loops, which are conserved across species. The central pacemaker, located in the suprachiasmatic nucleus (SCN) of the brain, coordinates peripheral clocks in tissues throughout the body, ensuring temporal coherence and optimal physiological function. Recent advances in genomics and next-generation sequencing have elucidated the extensive role of circadian clocks in regulating metabolism, immune function, and other critical biological processes. Disruptions in circadian rhythms are linked to various health issues, including metabolic disorders and cancer. Understanding the molecular mechanisms and systemic interactions of circadian clocks offers promising avenues for therapeutic interventions aimed at restoring circadian homeostasis and improving overall health. This study synthesizes current understanding of the mechanisms and functions of circadian rhythms in animals, emphasizing their evolutionary and physiological importance. Keywords Circadian rhythms; Molecular clocks; Suprachiasmatic nucleus; Transcriptional-translational feedback loops; Metabolic regulation 1 Introduction Circadian rhythms are intrinsic, approximately 24-hour cycles that regulate various physiological and behavioral processes in almost all living organisms, from bacteria to humans. These rhythms are driven by an internal time-keeping system known as the circadian clock, which synchronizes biological functions with the external environment, primarily the light-dark cycle (Sanchez et al., 2021; Zheng et al., 2021; Yang et al., 2023). The circadian clock is crucial for maintaining homeostasis, influencing sleep-wake cycles, feeding behaviors, hormone release, and other vital functions (King and Takahashi, 2000; Bloch et al., 2013). Disruptions in circadian rhythms can lead to adverse health effects, including metabolic disorders, impaired cognitive function, and increased susceptibility to chronic diseases (Goede et al., 2018; Meli, 2018). The study of circadian rhythms has a rich history, dating back to the early observations of daily leaf movements in plants by Jean-Jacques d'Ortous de Mairan in the 18th century. However, significant advancements were made in the 20th century with the discovery of the suprachiasmatic nucleus (SCN) in the hypothalamus as the master circadian clock in mammals (King and Takahashi, 2000; Sanchez et al., 2021). The identification of core clock genes such as Clock, Bmal1, Period, and Timeless in drosophila and mammals marked a new era in circadian biology, providing a molecular framework for understanding circadian mechanisms (King and Takahashi, 2000; Panda, 2016). The awarding of the Nobel Prize in Physiology or Medicine in 2017 to Jeffrey C. Hall, Michael Rosbash, and Michael W. Young for their work on the molecular mechanisms of circadian rhythms underscored the importance of this field (Meli, 2018). This study provides a comprehensive overview of the mechanisms and functions of circadian rhythms in animals. This study explores the molecular and genetic underpinnings of circadian clocks, their role in regulating physiological processes, and the impact of environmental factors on circadian regulation; additionally, discusses the adaptive significance of circadian rhythms in various animal species and the consequences of circadian disruption on health and fitness. By synthesizing current research findings, this study seeks to highlight the complexity and importance of circadian rhythms, offering insights into potential areas for future investigation.
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