[Insulin sensitivity and hepatic glucose production: nycthemeral variations]

Glucose tolerance is subject to circadian variations and has been found to be better in the morning or afternoon. An increase in plasma glucose and insulin concentrations has recently been observed between 5 and 9 a.m. These physiological changes correspond to an increase in insulin requirements in patients with insulin-dependent diabetes mellitus (the "dawn phenomenon"). In patients with non-insulin-dependent diabetes mellitus, the circadian rhythm of insulin secretion is suppressed. Studies of insulin sensitivity during a hyperinsulinaemic hyperglycaemic clamp (a supraphysiological experimental condition) indicated that: i) insulin-induced glucose utilization is decreased in the morning according to a circadian rhythm; ii) this change results from an increase in hepatic glucose production but not from a decrease in glucose uptake; and iii) a circadian rhythm is also observed for plasma free fatty acids and cortisol concentrations, which could account in part for the circadian changes in insulin sensitivity. Recognition of these circadian changes has implications for the treatment of non-insulin-dependent diabetes mellitus.