Functional circadian clocks are essential for fitness in diverse ecosystems, facilitating detection of predictable light-dark and temperature cycles. The molecular basis of endogenous clocks is variable across the tree of life, but it has one omnipresent attribute: natural genetic diversity that manifests as variation for instance in circadian period length around the hypothesised optimum of 24 hours. Latitudinal variation in photoperiod alone is unlikely to account for the vast diversity documented in varied organisms, but we have yet to achieve a solid understanding of the interplay between clock variability and natural selection. Recent circadian studies sampling populations have drawn attention to the hierarchical structure of genetic diversity in the wild, unveiling pronounced genetic variation even on a scale of metres.