PF-670462 is a potent inhibitor of the CK1 isoforms CK1ε and CK1δ (IC50 = 7.7 and 14 nM, respectively). Inhibition of CKIepsilon yields a perturbation of oscillator function that forestalls light as a zeitgeber, and they demonstrate that pharmacological tools such as PF-670462 may yield valuable insight into clock function. Casein kinase Iepsilon (CKIepsilon) is an essential component of the biological clock, phosphorylating PER proteins, and in doing so regulating their turnover and nuclear entry in oscillator cells of the suprachiasmatic nucleus (SCN).