Abstract
Caloric restriction prolongs lifespan and preserves health across species, with feeding times synchronized to day-night cycles further maximizing benefits. However, the mechanisms linking diet, diurnal rhythms, and lifespan remain unclear. In mice, the time point most strongly tied to dietary effects on lifespan coincides with the peak of glucocorticoid secretion (ZT12, lights-off). Caloric restriction raises circulating glucocorticoid hormone levels, implicating these signals as candidate mediators for its benefits. Here we show that in the liver, the glucocorticoid receptor (GR) is required for the metabolic response to caloric restriction. Hepatocyte-specific GR mutant males fail to mount this response, indicating that increased glucocorticoid amplitude is necessary for the adaptation. Using multiomics, we find that nutrient deprivation elicits a nuclear switch from active STAT signaling to increased FOXO1 activity, enabling GR to activate diet-specific gene expression programs. Our results suggest that glucocorticoid rhythms are crucial for caloric restriction-induced metabolic reprogramming.