If your blood sugar levels get higher when you haven’t eaten for 2 hours or more, this may be a signal to talk with your healthcare team about prediabetes, diabetes, or possibly another health issue that’s raising your glucose levels.

When you eat, your body breaks down your food into an energy source called glucose, or blood sugar. Most people will experience a drop in blood sugar when they don’t eat or drink beverages with calories for several hours.

But for some, their blood sugar still stays high for a long time. This is called fasting hyperglycemia.

Fasting hyperglycemia can happen when you have some health conditions, such as diabetes or difficulties with your pancreas or endocrine system.

Here’s what to know about fasting hyperglycemia, why you may experience it, and what you can do about it.

Hyperglycemia is a higher-than-normal blood sugar level. Fasting hyperglycemia is when your blood sugar levels stay high even when you haven’t eaten for a long time. Doctors will sometimes have you fast (not eat) for several hours or overnight so they can see what your blood sugar level will be.

Over time, hyperglycemia can damage your body’s tissues and organs. That’s because the sugar can damage your nerves, kidneys, eyes, veins, or blood vessels.

Doctors check your fasting blood sugar levels to figure out if you have hyperglycemia caused by prediabetes, type 1 diabetes, type 2 diabetes, or other conditions.

A doctor will use these glucose levels to help figure out whether higher blood sugars are a symptom of developing diabetes:

  • normal: 99 milligrams per deciliter (mg/dL) or below
  • prediabetes: 100–125 mg/dL
  • diabetes: 126 mg/dL or higher

But for some people, high blood sugar levels aren’t due to prediabetes or diabetes.

Usually, when you fast, your body uses the sugar in your blood to fuel your cells and organs. That causes it to dip. If it falls too low, you can feel dizzy, cranky, sweaty, or have blurred vision or heart palpitations.

But sometimes, your blood sugar goes up even when you fast.

It could be because your body isn’t making enough insulin, a hormone that allows your cells to absorb the sugar. Or it could be because your body has become insulin resistant. It could also be because your body is making too much sugar.

This could be because a condition such as diabetes or prediabetes is interfering with your body’s ability to use sugar. Other conditions can also cause it.

Other causes of hyperglycemia include:

Symptoms of hyperglycemia can include:

  • fatigue
  • thirst
  • blurry vision
  • headache
  • the need to urinate more often

Some people have no symptoms other than the high blood sugar level shown on a test.

Fasting hyperglycemia doesn’t always mean you have diabetes.

You can have it for other reasons, such as a hormone imbalance, a problem with your pancreas, illness, surgery, or another reason. It’s important to work with a doctor to figure out the cause.

Fasting doesn’t usually cause high blood sugar, but it’s possible.

If you already have diabetes, you may experience an early-morning increase in blood sugar, known as the dawn phenomenon. That’s when a surge of nighttime hormones causes a blood sugar spike in people with diabetes, but this doesn’t affect those without diabetes.

If doctors find that you have fasting hyperglycemia, they’ll try to figure out why. The cause determines your treatment.

For instance, if you have type 1 diabetes, they’ll recommend a nutrition plan and probably prescribe insulin to help your body use blood sugar properly. They’ll give you ways to check your blood sugar so that it stays in a normal range.

If you have type 2 diabetes, they’ll likely suggest lifestyle and diet changes and may give you different types of medication. They’ll also give you ways to check your blood sugar levels to figure out if treatment is working.

They may also give you more tests to figure out if high blood sugar damaged other parts of your body. Then, they’ll help you decide whether you need further treatment.

Other conditions may require different types of treatment.

Most people don’t have high blood sugar levels after several hours of fasting. When you have high blood sugar levels even though you haven’t eaten or drunk calorie-containing beverages, it’s called fasting hyperglycemia.

If doctors think you have a medical condition, such as diabetes, they may have you take a test to figure out if you have fasting hyperglycemia. You can have it for reasons such as prediabetes, diabetes, the use of some medications, or hormone imbalances.

High blood sugar levels can damage your cells, tissues, and organs over time. It’s important to know if you have hyperglycemia so you can work with a doctor to figure out causes and treatment.