Morning insulin resistance has frequently been invoked to explain early-morning increases in both basal and breakfast-associated insulin requirements in diabetic patients. This increase in insulin requirements and plasma glucose from 0600 to 0900, when compared with midnight to 0600, has been termed the dawn phenomenon. We believe that the increased need for insulin in the morning has been misinterpreted. Data are reviewed that suggest the major perturbation overnight is a sleep-associated fall in hepatic glucose output, with a return to basal production rates on arousal in the morning. Moreover, the apparent increased insulin requirement for breakfast compared with lunch or supper (meal phenomenon) appears to be related more to lack of residual insulin effect from a preceding meal than to any putative morning insulin resistance. Thus, we found little evidence to support morning insulin resistance as a cause of either the dawn phenomenon (more appropriately designated the sleep phenomenon) or the meal phenomenon. A proper understanding of these phenomena is essential to the management of diabetic patients receiving insulin.

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