The dawn phenomenon, a tendency for glucose to rise between 0500 and 0800 h in subjects with diabetes, is also reflected as an increase in insulin required to maintain normoglycemia during closed-loop insulin infusion. Individuals without diabetes have minimal or absent rises in early morning glucose. To test the hypothesis that the absence of early morning glucose increases in subjects without diabetes is due to an increase in insulin levels, we measured insulin levels from 2400 to 0800 h in four male and two female volunteers. Subjects were on an unrestricted diet with three main meals and one bedtime snack at 2100 h. Blood samples were collected continuously in hourly pools by a constant-rate withdrawal pump. We observed the following: (1) hourly integrated concentration of glucose was stable from 2400 to 0800 h (range of mean plasma values, 94.5–97.3 mg/dl), and (2) hourly integrated concentration of insulin increased from the 0300–0400 (4.6 μU/ml) to the 0700–0800-h pool (6.2 μUμml) (P < 0.05). The observed increase in insulin in the early morning hours despite stable levels of glucose indicates a temporally increased insulin need in nondiabetic individuals similar to that found in individuals with diabetes. The mechanism underlying this increased insulin need may be similar in diabetes and nondiabetes, with the ensuing rise in glucose being dependent on the availability of compensatory insulin.