Consistent variations in glucose regulation across the 24-hour cycle are present in normal subjects. These diurnal variations are altered in various states of impaired glucose tolerance (aging, obesity, diabetes). Changes in insulin secretion, clearance and/or action across the day have been demonstrated. Studies in subjects receiving continuous intravenous glucose infusion have shown that major alterations of glucose tolerance occur during sleep and that sleep quality markedly influences glucose utilization. Diurnal variations in glucose tolerance result from the alternation of wake and sleep states as well as from intrinsic effects of circadian rhythmicity. The important roles of physiological variations in levels of counterregulatory hormones which are markedly dependent on sleep (i.e. growth hormone) or circadian rhythmicity (i.e. cortisol) have only begun to be appreciated. The modulatory effects of sleep and circadian rhythmicity on glucose regulation may have important clinical implications for the diagnosis and treatment of abnormalities of carbohydrate metabolism.