The circadian clock allows organisms to accurately predict the earth's rotation and modify their behaviour as a result. Genetic analyses in a variety of organisms have defined a mechanism based largely on gene expression feedback loops. However, as we delve more deeply into the mechanisms of circadian timekeeping we are discovering that post-translational mechanisms play a key role in defining the character of the clock. We are also discovering that these modifications are inextricably linked to cellular metabolism including redox homeostasis. A robust circadian oscillation in the redox status of the peroxiredoxins (a major class of cellular antioxidants) was recently shown to be remarkably conserved from archaea and cyanobacteria all the way to plants and animals. Furthermore, recent findings indicate that cellular redox status is not only coupled to canonical circadian gene expression pathways but also to a non-canonical transcript-independent circadian clock. The redox rhythms observed in peroxiredoxins in the absence of canonical clock mechanisms may hint at the nature of this new and hitherto unknown aspect of circadian timekeeping.