In all living organisms, one of the most indispensable biological functions is the circadian clock (suprachiasmatic nuclei; SCN), which acts like a multifunction timer to regulate homeostatic systems such as sleep and activity, hormone levels, appetite, and other bodily functions with 24. h cycles. Circadian rhythms regulate diverse physiologic processes, including homeostatic functions of steroid hormones and their receptors. Perturbations of these rhythms are associated with pathogenic conditions such as depression, diabetes and cancer. Clock genes are identified as the genes that ultimately control a vast array of circadian rhythms in physiology and behavior. Clock gene regulates several diseases such as cancer, metabolic syndrome and sleep etc. CLOCK mutation affects the expression of rhythmic genes in wild-type (WT) tissue, but also affects that of non-rhythmic genes. On the other hand, the change of the drug pharmacodynamic and pharmacokinetic (PK/PD) parameters are influenced by not only inter-individual variability but also intra-individual variabilities of medications. Identification of a rhythmic marker for selecting dosing time will lead to improved progress and diffusion of chronopharmacotherapy. The mechanisms underlying chronopharmacological findings should be clarified from viewpoint of clock genes. On the other hand, several drugs have an effect on molecular clock. Thus, the knowledge of intra- and inter-individual variability of molecular clock should be applied for the clinical practice. Therefore, we introduce the regulatory system of biological rhythm from viewpoints of clock genes and the possibility of pharmacotherapy based on the intra- and inter-individual variability of clock genes.
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