The human circadian system is organized as a multilevel oscillator network. The master circadian clock (pacemaker) is located in the suprachiasmatic nucleus (SCN) of the hypothalamus, where it receives photic information from the ganglion cells in the retina. The process synchronizes the SCN clock to the solar day. The SCN subsequently integrates and synchronizes peripheral circadian clocks in metabolically active tissues, such as pancreatic beta cells, skeletal myocytes and hepatocytes, to the solar day by employing a combination of neuronal, behavioral and endocrine outputs. Subsequently, intracellular circadian clocks in metabolic tissues exert physiological control over glucose metabolism through regulation of insulin secretion (beta cell), insulin-mediated glucose uptake (skeletal muscle) and insulin-mediated hepatic glucose production (hepatocytes).
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