Circadian System, Volume 137 in the Advances in Protein Chemistry and Structural Biology series, highlights new advances in the field, with this new volume presenting interesting chapters that comprehensively cover Circadian Systems Genes and Their Importance for Human Health, Single nucleotide polymorphisms (SNPs) in circadian genes: Impact on Gene Function and Phenotype, Relationship between circadian rhythm and diseases: sex hormones impact, Advances in circadian clock regulation of reproduction, Disruption of the clock gene expression in central and peripheral circadian oscillators by maternal overnutrition in the Oryctolagus cuniculus, COVID-19 disease management in aspect of Circadian system, and more. Other sections cover Molecular Characterization of Circadian Gene Expression and Its Correlation with Survival Percentage in Colorectal Cancer Patients, Identifying circadian gene signature that estimates overall survival from colon adenocarcinoma, The role of environmental signals in the expression of rhythmic cardiac proteins and their influence on cardiac pathologies, and much more.