Background: Circadian rhythms are fundamental to regulating metabolic processes and cardiovascular functions. Phosphorylated PERIOD2 (PER2) is a key factor in determining the period of the mammalian circadian clock. Moreover, casein kinase 1\epsilon (CK1\epsilon) primes the PER2 phosphoswitch and its stability. While diabetes contributes to the disorder of the circadian system, changes in PER2 forms and their regulatory mechanisms during diabetes remain unclear. In this study, we examined the impact of diabetes on PER2 and CK1\epsilon signaling in the heart to determine the potential mechanism between them. Methods: A Type-1 diabetic rat model was established by intraperitoneally injecting rats with streptozotocin. General characteristics, cardiac function, histology, serum biochemistry, apoptosis index and circadian rhythm were analyzed in controls and diabetic rats treated with or without PF-670462 (a CK1\epsilon inhibitor). A high-glucose model was created with H9c2 cells and treated with PF-670462 and PER2 siRNA. Cell viability, LDH release, dead/live rate and histology were determined to assess cellular injuries. RT-PCR and Western blot were used to evaluate the expression of PER2, CK1\epsilon, phosphorylated PER2, and immunofluorescence (IF) was employed to determine PER2’s location. Results: STZ-induced diabetes prolonged PER’s period and upregulated the expression of CK1\epsilon and phosphorylated PER2 compared to the controls. Inhibiting CK1\epsilon and PER2 with PF-670462 downregulated the phosphorylation at Ser662 and the nuclear entry of PER2 in high glucose conditions. In addition, pharmacologically or genetically suppressing PER2 mitigated high-glucose-instigated myocardial injury. Conclusions: Diabetes compromised PER2 in association with activated CK1\epsilon signaling. Targeting CK1\epsilon-regulated PER2 alleviates myocardial injuries in the presence of high glucose.