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Abstract The diversification of biodiversity progresses as newly evolving species adapt to occupy available niche space – including the temporal dimension. Throughout the history of life, animal species adapted to occupy specific regions, or all regions (cathemerality), of the day-night temporal spectrum. While adaptive radiation theory is predicted to drive much of the global proliferation of biodiversity, the role that the day-night temporal dimension plays in offering ecological opportunity for lineages to diversify remains fundamentally neglected. Using a dataset spanning 19,940 species from across all four main tetrapod lineages (amphibians, squamates (restricted to lizards), mammals and archosaurs), we perform the very first evolutionarily standardised temporal niche diversification analysis and the first to include substantial number of ectotherms. We examine temporal niche as a source of ecological opportunity for adaptive radiation and contrast how lineages have leveraged ecological opportunity spread across 24h time. Findings revealed that tetrapods most frequently transitioned towards diurnality, suggesting a general opening of ecological opportunity in diurnal niche space since the K-Pg mass extinction. Moreover, amphibians are faster and more flexible than amniotes in temporal niche evolution, supported by a relatively higher speed of temporal niche transition and by spending relatively more in time cathemerality, potentially compensating for limitations in geographic dispersal. This interpretation suggests the day-night temporal niche dimension interacts with the geographic dimension, as it exhibits processes which unfold in parallel to (independent of) the geographic dimension as well as processes which unfold in response to what occurs in the geographic dimension.