Metabolic effects of short-term elevations of plasma cortisol are more pronounced in the evening than in the morning.

To determine whether elevations of cortisol levels have more pronounced effects on glucose levels and insulin secretion in the evening (at the trough of the daily rhythm) or in the morning (at the peak of the rhythm), nine normal men each participated in four studies performed in random order. In all studies, endogenous cortisol levels were suppressed by metyrapone administration, and caloric intake was exclusively under the form of a constant glucose infusion. The daily cortisol elevation was restored by administration of hydrocortisone (or placebo) either at 0500 h or at 1700 h. In each study, plasma levels of glucose, insulin, C-peptide, and cortisol were measured at 20-min intervals for 32 h. The initial effect of the hydrocortisone-induced cortisol pulse was a short-term inhibition of insulin secretion without concomitant glucose changes and was similar in the evening and in the morning. At both times of day, starting 4-6 h after hydrocortisone ingestion, glucose levels increased and remained higher than under placebo for at least 12 h. This delayed hyperglycemic effect was minimal in the morning but much more pronounced in the evening, when it was associated with robust increases in serum insulin and insulin secretion and with a 30% decrease in insulin clearance. Thus, elevations of evening cortisol levels could contribute to alterations in glucose tolerance, insulin sensitivity, and insulin secretion.