Childhood obesity remains one of the most important public health challenges worldwide, resulting from the complex interaction of biological, behavioral, environmental, and socioeconomic factors. Although substantial advances have been achieved in nutritional science, increasing evidence suggests that metabolic regulation is influenced not only by dietary composition but also by biological timing. This narrative conceptual review synthesizes current knowledge from chronobiology, chrononutrition, pediatric endocrinology, nutritional physiology, and behavioral nutrition to examine how early-day eating patterns may influence metabolic regulation during childhood. The available evidence indicates that metabolic responses emerge from the coordinated interaction among circadian organization, endocrine signaling, dietary quality, feeding behavior, and developmental physiology rather than from isolated nutritional variables alone. Based on this integrated interpretation, this review proposes the concept of the Early-Day Metabolic Window (EMW), defined as the physiological period immediately following overnight fasting during which circadian synchronization, endocrine responsiveness, metabolic flexibility, and nutrient sensing converge during the transition toward daytime metabolism. Building upon this concept, we present the Costa Reset Conceptual Model, an integrative physiological framework designed to organize current evidence into a unified interpretation of early-day metabolic regulation. Rather than proposing a new biological mechanism or a specific dietary intervention, the model provides a conceptual structure through which biological timing and nutritional exposures may be interpreted as interacting determinants of pediatric metabolic regulation. Although the proposed framework remains conceptual and requires prospective experimental validation, it generates biologically plausible and testable hypotheses that may guide future clinical and translational research. Integrating chrononutrition and temporal nutritional physiology into pediatric nutritional science may contribute to a more comprehensive understanding of childhood metabolic health and support future chrononutrition-based preventive and therapeutic strategies.
