Fasting glucose is the level of sugar in your blood after you haven’t eaten for at least 8 hours. It’s one of the most common blood tests in medicine, used to screen for diabetes and prediabetes. A normal fasting glucose falls below 100 mg/dL, while levels between 100 and 125 mg/dL signal prediabetes, and 126 mg/dL or higher on two separate tests indicates diabetes.
How Your Body Maintains Blood Sugar Overnight
When you stop eating, your body doesn’t just run out of fuel. Your liver steps in as a glucose reservoir, slowly releasing stored sugar into your bloodstream to keep your brain and organs running. Over the first several hours of fasting, the liver acts as a buffer, preventing blood sugar from dropping too low.
Two hormones drive this process. Insulin, produced after meals, lowers blood sugar by moving glucose into cells. Glucagon does the opposite: when blood sugar dips, the pancreas releases glucagon to signal the liver to release more glucose. These two hormones work in constant balance to keep your levels steady. If fasting continues for many hours, the liver begins manufacturing new glucose from amino acids and other building blocks through a process called gluconeogenesis. Over longer periods, growth hormone and cortisol also kick in, shifting your body toward burning fat so it can conserve its remaining glucose.
A fasting glucose test captures how well this system works. If your reading is high after 8 or more hours without food, it means your body is struggling to regulate blood sugar on its own, usually because insulin isn’t working effectively or your pancreas isn’t producing enough of it.
What the Numbers Mean
Fasting glucose results fall into three categories:
- Normal: below 100 mg/dL (5.6 mmol/L)
- Prediabetes (impaired fasting glucose): 100 to 125 mg/dL (5.6 to 6.9 mmol/L)
- Diabetes: 126 mg/dL (7.0 mmol/L) or higher on two separate occasions
Prediabetes is not a harmless waiting room before diabetes. A large study published in Diabetes Care found that once fasting glucose rises above 100 mg/dL, the risk of cardiovascular events begins climbing. People with levels between 110 and 125 mg/dL had a 10 to 20% higher risk of heart disease compared to those with normal levels, and those in the highest glucose categories faced double the risk or more. The greatest increases were seen for heart attacks and strokes caused by blood clots. Elevated glucose damages blood vessel walls, speeds up plaque buildup in arteries, and makes those plaques more likely to rupture.
A single high reading doesn’t confirm a diagnosis. Doctors typically repeat the test or pair it with other tests before making any determination.
How Fasting Glucose Compares to A1C
Your fasting glucose is a snapshot of a single moment. Your A1C (also called hemoglobin A1C) reflects your average blood sugar over the previous three months. Both are used to diagnose diabetes and prediabetes, but they answer different questions.
Fasting glucose tells you how well your body handles blood sugar regulation overnight. A1C gives a broader picture of how your blood sugar behaves across weeks, including after meals. Because blood sugar fluctuates throughout the day, one test can look normal while the other flags a problem. That’s why many doctors order both. You don’t need to fast for an A1C test, which makes it more convenient, but it doesn’t replace the information a fasting test provides about your baseline regulation.
How to Prepare for the Test
The preparation is straightforward: don’t eat or drink anything except water for 8 to 12 hours before your blood draw. Your provider will tell you the exact window. Most people schedule the test first thing in the morning so the fasting period overlaps with sleep. Water is fine and actually helpful, since dehydration concentrates your blood sugar and can push your reading higher than it truly is.
What Can Throw Off Your Results
Several things beyond food can affect a fasting glucose reading, and they’re worth knowing if your result comes back unexpectedly high.
Poor sleep is one of the most common culprits. Even a single night of inadequate sleep reduces your body’s ability to use insulin effectively, which can raise your morning glucose. Stress has a similar effect. Physical or emotional stress triggers cortisol and adrenaline, both of which push blood sugar up. Even something like a bad sunburn can cause enough physical stress to elevate your levels. Dehydration matters too: less water in your body means the same amount of glucose is dissolved in less fluid, producing a higher concentration on the test.
If any of these factors were present the night before your test, mention it to your provider. A retest under better conditions can give a more accurate picture.
Why Morning Readings Can Be Surprisingly High
Some people, especially those already managing diabetes, notice that their blood sugar is higher when they wake up than it was at bedtime. This often comes from what’s called the dawn phenomenon. In the early morning hours, your body releases a surge of hormones (cortisol, growth hormone, and others) that naturally counteract insulin. If your body can’t produce enough insulin to compensate, your blood sugar rises before you’ve eaten anything.
The dawn phenomenon is common and distinct from another, less frequent pattern called the Somogyi effect, where blood sugar drops too low overnight (often from too much insulin the evening before) and then rebounds sharply as the body overcorrects. The dawn phenomenon is far more common. If your fasting numbers are consistently elevated despite good eating habits, this hormonal morning surge is a likely explanation worth discussing with your provider.