Circadian genes influence a variety of biological processes that are important in prostate tumorigenesis including metabolism. To determine if variants in circadian genes alter prostate cancer risk, we genotyped five variants in five circadian genes in a population-based case-control study conducted in China (187 cases and 242 controls). These variants included CRY2 rs1401417:G>C, CSNK1E rs1005473:A>C, NPAS2 rs2305160:G>A, PER1 rs2585405:G>C and PER3 54-bp repeat length variant. Men with the cryptochrome 2 (CRY2)-variant C allele had a significant 1.7-fold increased prostate cancer risk (95% confidence interval (CI), 1.1-2.7) relative to those with the GG genotype. This risk increased to 4.1-fold (95% CI, 2.2-8.0) in men who also had greater insulin resistance (IR) as compared to men with the GG genotype and less IR. In contrast, among men with less IR, the NPAS2-variant A allele was associated with decreased prostate cancer risk (odds ratio=0.5, 95% CI, 0.3-1.0) as compared to the GG genotype. Our findings, although in need of confirmation, suggest that variations in circadian genes may alter prostate cancer risk and some biological processes may modify this effect.