A news article by Charles Schmidt propagates the Diversity Outbred mouse model to facilitate the extrapolation of toxicology findings to humans. In this regard, we would like to highlight two sources of animal diversity that could be relevant: 1 ) the differential—and genetically codetermined—circadian propensity for activity and rest with which the mice are born, and 2 ) the differential stability of circadian rhythms in later life stages of mice, to which perinatal photoperiods may contribute.