Plasma cortisol concentrations increase approximately three- to five-fold during sleep in healthy humans. To determine the effects of the normal nocturnal rise in cortisol on carbohydrate and fat metabolism independent of changes in endogenous insulin secretion, we studied the disposition of a mixed meal in individuals with insulin-dependent diabetes mellitus (IDDM) in whom the normal nocturnal rise in cortisol had been either prevented or mimicked by using metyrapone and a constant or variable hydrocortisone infusion. Insulin was infused intravenously on both occasions in amounts sufficient to create relative postprandial insulin deficiency. The nocturnal rise in cortisol resulted in an approximately 30 mg/dl greater (P < 0.001) peak postprandial glycemic excursion due to greater (P < 0.01) systemic glucose appearance and inappropriately low (P < 0.05) tissue glucose uptake. The latter was most evident when postprandial glucose concentrations in the presence and absence of the nocturnal rise in cortisol were matched by means of an exogenous glucose infusion to avoid the confounding effects of differences in glycemia. The nocturnal rise in cortisol also resulted in increased (P < 0.01) incorporation of 14CO2 into glucose (an index of gluconeogenesis), decreased (P < 0.05) carbohydrate oxidation, and increased (P < 0.05) rates of palmitate appearance, lipid oxidation, and beta-hydroxybutyrate concentrations. Thus the normal nocturnal rise in cortisol, independent of changes in insulin secretion, is an important regulator of postabsorptive and postprandial carbohydrate, fat, and ketone body metabolism in humans.