Powered by molecular tools, circadian research rapidly reveals a key role of biological rhythms in physiology and health, but clock function is poorly understood in its organismic and environmental context. Because clocks integrate multiple rhythmic processes, they need to be finely adjusted to environmental conditions. To address how genes and environment shape timekeeping, I combine frontline molecular tools and field-based approaches in the study of an avian model species in ecology and evolution, the Great Tit (Parus major). This research thrives on the complementarity of expertise and facilities of my host institution and the strengths that I bring. The project will lay the foundations for a long-term study system of biological time-keeping across levels of biological organization that will advance our understanding of the ways clocks aid fitness and health. Specifically, I will use cross-fostering designs and telemetry in the wild to test for early-environment effects on daily timing (’chronotype’), I will examine birds in captivity to quantify endogenous clock features, and use high-throughput molecular profiling technologies to advance the genotype-phenotype link in chronobiology.The research is based on my earlier findings of highly heritable, but nonetheless dynamic circadian clocks, which I can now greatly expand through the combined assets of the field station SCENE and the cutting-edge tools and analytical expertise for integrated analysis of multi-level ‘omic data at the University of Glasgow. The hosting Institute of Biodiversity, Animal Health and Comparative Medicine, is an internationally leading centre of multidisciplinary research with the mission of using ecological expertise to address urgent problems of today. With its unique focus and combination of approaches, the proposed research will bring novel perspectives to the institute and will boost my integration, while contributing significantly to understanding circadian clocks in real life.