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Many animals have strict diel activity patterns, with unique adaptations for either diurnal or nocturnal activity. Diel activity is phylogenetically conserved, yet evolutionary shifts in diel activity occur and lead to important changes in an organism's morphology, physiology, and behavior. We use phylogenetic comparative methods to examine the evolutionary history of diel activity in skinks, one of the largest families of terrestrial vertebrates. We examine how diel patterns are associated with microhabitat, ambient temperatures, and morphology. We found support for a nondiurnal ancestral skink. Strict diurnality in crown group skinks only evolved during the Paleogene. Nocturnal habits are associated with fossorial activity, limb reduction and loss, and warm temperatures. Our results shed light on the evolution of diel activity patterns in a large radiation of terrestrial ectotherms and reveal how both intrinsic biotic and extrinsic abiotic factors can shape the evolution of animal activity patterns. [ABSTRACT FROM AUTHOR]