Urbanization provides a compelling opportunity to study evolutionary processes; by dramatically changing the environment, cities expose organisms to profoundly distinct selective pressures compared to their natural environment. Here, I will use urbanisation to study the evolution of biological clocks. Circadian clocks allow organisms to anticipate daily events and are ubiquitous in nature. They “tick” at different rates in different individuals, thus showing variation in their properties, such as period length, which is highly heritable. But evolution of clocks is only possible if this variation is also associated with fitness differences. Currently, however, selection on clocks remain poorly understood. Here I will use city/forest clock differences to test the prediction that altered environmental cues of cities select for weaker and faster clocks. I will use great tits (Parus major), a songbird that breeds in both environments. First, I will explore whether city/forest clock differences are either genetic or due to their pre-hatching or post-hatching environment. For this, I will use experimental manipulations, cross-fostering at the egg stage and carry out common-garden measurements. Then, I will assess whether clocks are locally adapted by producing a genomic dataset to assess urban signatures of selection, with a particular focus on core clock-related genes. Finally, I will assess fitness consequences of variation in clocks by relating variation in clock properties to breeding success and survival. I will use cultures of skin fibroblasts to measure clock properties of wild birds and, this way, obtain data on clocks and fitness in free-living individuals, which would otherwise be impossible. This project uniquely combines techniques from evolutionary ecology, genomics and chronobiology to study clocks under real-world selective pressures. This will not only shed light on the micro-evolution of circadian clocks but on our understanding to what extent organisms can adapt to urban environments.