Updated May 18, 2006

Dawn Phenomenon

Very high blood glucose in the early morning due to the release of certain hormones in the middle of the night. The body makes certain hormones called counterregulatory hormones, which work against the action of insulin. These hormones, which include glucagon, epinephrine, growth hormone, and cortisol, raise blood glucose levels, when needed, by signaling the liver to release more glucose and by inhibiting glucose utilization throughout the body.

In the middle of the night, there is a surge in the amount of growth hormone the body releases, followed by a surge in cortisol, which effectively cranks up glucose production in the liver, presumably to prepare the body for daytime activity after a period of fasting. In people who don't have diabetes, these processes are offset by increased insulin secretion by the pancreas, so blood glucose levels remain relatively stable. However, in people with Type 1 diabetes, whose pancreases don't make insulin, and in people with Type 2 diabetes, whose livers may not respond to insulin well enough to stop glucose production, changes in glucose metabolism during sleep can have a profound effect on morning blood glucose levels. Typically, the blood glucose level rises between 4 AM and 8 AM.

It is important to realize that high morning blood sugar may be caused by something else: the body's rebound from low blood glucose levels at night. Rebound hyperglycemia, also caused by the release of counterregulatory hormones, represents the body's defense mechanism against low blood sugar. The only way to tell the two phenomena apart is to check your blood glucose level in the middle of the night (around 3 AM). If your blood sugar is high, you are probably experiencing the dawn phenomenon; if it is low, rebound hyperglycemia is probably at work.

If you experience high blood glucose levels when you wake up, talk with your health-care team. They can recommend further testing and changes in your meal, exercise, or insulin regimen to alleviate the problem.

This article was written by Robert S. Dinsmoor, a Contributing Editor of Diabetes Self-Management.

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