When you go to bed, your blood sugar reading is 110, but when you wake up in the morning, it has shot up to 150. Why does this happen?

To understand how blood sugar levels can rise overnight without your eating anything, we have to look at where glucose comes from — and where it goes — while we sleep.

During the day, the carbohydrates we eat are digested into glucose and absorbed into the bloodstream. Some of this glucose goes to the liver, where it is stored for later use.

At night, while we are asleep, the liver releases glucose into the bloodstream. The liver acts as our glucose warehouse and keeps us supplied until we eat breakfast. The amount of glucose being used is matched by the amount of glucose being released by the liver, so blood sugar levels should remain constant.

A rise in blood sugar level between approximately 3 A.M. and the time you wake up is called the “dawn phenomenon.”

The liver is supposed to release just enough glucose to replace what is being used, and insulin works as the messenger to tell the liver how much is enough. But if there's not enough insulin (as with type 1 diabetes), or if there's enough insulin but it cannot communicate its message to the liver (as with type 2 diabetes), the liver starts to release glucose much too quickly. In addition, levels of hormones such as cortisol begin to increase in the early morning hours, which can contribute to altered insulin sensitivity.

The result? Blood sugar levels rise. This is why blood sugar levels can go up between the time you go to bed and the time you wake up.

You might be able to make changes in the timing of your meals, medications, or insulin injections to help prevent dawn phenomenon. First, keep a detailed record of what's happening in the evening and in the morning, including your blood sugar levels, eating habits, medications or insulin, and physical activity. Then, talk to your health care provider or diabetes educator about changes you can make to prevent dawn phenomenon.

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