Are humans cathemeral? The diurnal activity model of Homo sapiens challenged

Humans are traditionally classified as diurnal primates, although field evidence from small-scale societies complicates a rigid behavioral reading of that designation. Here, I evaluate the Human Diel Flexibility hypothesis-the idea that Homo sapiens retains diurnal circadian physiology while expressing unusually flexible 24-hour activity for a large-bodied anthropoid. In study 1, I conducted actigraphy-based comparisons of diel (i.e., 24 hour) activity patterns across 10 primate species, including Hadza hunter-gatherers, gibbons (Hylobates), and seven lemur species. Functional linear modeling and Bayesian multivariate analysis revealed that Hadza individuals exhibit elevated night-time activity and a significantly lower day-to-night time ratio (DNTR) than gibbons, overlapping with some lemur genera in this dataset. In study 2, I compiled a comparative dataset of day-to-night activity ratios from 24 primate species and conducted phylogenetic generalized least squares analyses. I found that humans maintain a relatively high cathemerality index in this comparative framework by ecological and morphological variables and that endocranial volume is a significant positive predictor of diel flexibility. Evolutionary model comparisons are consistent with a two-regime Ornstein-Uhlenbeck model with distinct optima for humans and nonhuman primates, suggesting divergent selective pressures on human sleep-wake architecture. I propose the Social Sleep Theory to explain this pattern: through sociotechnological niche construction-group vigilance, fire, and shelter-humans evolved short, flexible, and high-quality sleep that enabled activity redistribution across the diel cycle. These findings challenge a rigid behavioral model of human diurnality while preserving the distinction between circadian physiology and activity expression and expand our understanding of how temporal niche plasticity has contributed to the ecological success of H. sapiens.