Sunlight regulates both physiological and behavioural rhythms and the ability to detect light and visually perceive the external environment has long been posited as a major catalyst in the radiation of animals (the ‘light-switch’ theory). But what happens when the lights go out? Animals that have shifted from an obligate diurnal lifestyle to an obligate nocturnal or aphotic environment represent a considerable evolutionary transition in sensory requirements. Tracking the associated phenotypic and genotypic changes to altered light environments over geological time scales can elucidate the key factors that drive adaptive radiations. Two case studies are presented that represent an adaptive shift to dim-light (nocturnal foraging social bees), and regressive evolution associated with cave habitats (blind subterranean water beetles). Both case studies exhibit structural change at the morphological and molecular level pertaining to their respective transition into novel photic environments and highlight the importance of addressing evolutionary questions from a comparative phylogenetic perspective.