Artificial light at night (ALAN) has been underexplored as an anthropogenic stressor compared to chemical and noise pollution and it can be considered a threat to biodiversity. Because organisms have evolved with the periodicity of light dark cycles, ALAN may affect multiple aspects of behaviour and physiology. Changes in behaviour and physiology associated with ALAN may have particularly strong effects early in life, when developmental trajectories are sensitive to stressful conditions. Yet, a major gap in knowledge involves the effects of ALAN during development, particularly in natural populations. Using birds as a model, we will perform the first study to comprehensively examine how exposure to ALAN affects physiological stress in nestlings in the short- and long-term. We will examine whether early life light exposure affects aging rate, as indicated by telomere dynamics. Finally, we will assess whether exposure to ALAN early in life has enduring effects on the phenotype and fitness (reproduction, survival) of adults. We will use experimental approaches, involving manipulations of light inside nestboxes and cross-fostering experiments. We will also collect correlational data, using a population exposed to heterogeneous light regimes. By elucidating effects of ALAN on developing organisms and assessing mitigating strategies (such as part-night lighting), our study will motivate action to cope with the consequences of living in lighted environments.