A normal fasting blood glucose level is 99 mg/dL (5.5 mmol/L) or below. This number comes from a blood draw taken after at least eight hours without food, typically first thing in the morning. If your result falls between 100 and 125 mg/dL, that’s considered prediabetes, and 126 mg/dL or higher points to diabetes.
The Three Ranges That Matter
The American Diabetes Association uses fasting plasma glucose to sort results into three categories, and these thresholds have remained unchanged in the most recent 2025 guidelines:
- Normal: 99 mg/dL (5.5 mmol/L) or below
- Prediabetes: 100 to 125 mg/dL (5.6 to 6.9 mmol/L)
- Diabetes: 126 mg/dL (7.0 mmol/L) or higher
A single reading in the diabetes range isn’t enough for a diagnosis on its own. You typically need two abnormal results, either from the same test on different days or from two different types of tests (such as a fasting glucose plus an A1C) taken at the same time. The exception is when blood sugar is very high and classic symptoms like excessive thirst and frequent urination are already present.
What Your Body Does Overnight
When you sleep and haven’t eaten for several hours, your body still needs a steady supply of glucose to fuel your brain and red blood cells. Your liver handles this job through two processes. First, it breaks down its stored form of sugar, called glycogen. Second, it builds brand-new glucose molecules from smaller building blocks like amino acids and lactate. After an overnight fast, these two pathways contribute roughly equally to the glucose circulating in your blood.
The whole system is orchestrated by hormones. As blood sugar dips, your pancreas releases glucagon and your adrenal glands release stress hormones, both of which tell the liver to push more glucose into the bloodstream. In a healthy person, insulin then fine-tunes the response so levels stay in that narrow range below 100 mg/dL. When that balancing act breaks down, fasting numbers start creeping up.
How to Prepare for the Test
Fasting means no food or caloric drinks for at least eight hours before your blood is drawn. Water is fine and encouraged, since dehydration can make a blood draw more difficult. Most people schedule the test for early morning so the fast happens during sleep. Ask your provider whether to take your regular medications beforehand, since some can affect results.
What Can Throw Off Your Results
A higher-than-expected fasting number doesn’t always mean prediabetes. Several temporary factors can push glucose up. Illness is a common one: your body releases hormones to fight infection, and those same hormones raise blood sugar. Stress does the same thing through a similar hormonal pathway. Even acute pain, something as simple as a bad sunburn, can trigger a short-term spike. If you were sick, sleep-deprived, or under unusual stress when you had your blood drawn, a repeat test under more typical conditions may give a more accurate picture.
Why Morning Readings Run High
Some people, especially those already managing diabetes, notice their fasting glucose is stubbornly elevated even when they ate well the night before. The most common explanation is the dawn phenomenon. In the hours between roughly 3 a.m. and 8 a.m., your body ramps up cortisol and growth hormone to prepare you for waking. These hormones signal the liver to release more glucose. In someone without diabetes, a matching bump in insulin keeps things balanced. In someone with diabetes, that insulin response is either too weak or absent, so blood sugar drifts up by morning. About half of people with type 1 or type 2 diabetes experience this.
A much rarer cause is the Somogyi effect. This happens when blood sugar drops too low during the night, often because of a missed meal or too much evening insulin. The body overcorrects by flooding the bloodstream with glucose, and the person wakes up with a high reading. The key difference: the dawn phenomenon involves a gradual rise in the early morning hours, while the Somogyi effect starts with a low point in the middle of the night. Checking blood sugar around 2 or 3 a.m. can help distinguish between the two.
Pregnancy Changes the Target
If you’re pregnant, the thresholds are tighter. The target for fasting glucose during pregnancy is below 95 mg/dL (5.3 mmol/L), whether you’ve been diagnosed with gestational diabetes or are managing pre-existing type 1 or type 2. For women with diabetes who are planning a pregnancy, the recommended pre-meal range is 80 to 110 mg/dL. These stricter numbers reflect the fact that even mildly elevated glucose can affect fetal development.
What a Prediabetes Result Means in Practice
Landing in the 100 to 125 mg/dL range is not a diabetes diagnosis. It’s a signal that your body is starting to struggle with blood sugar regulation. The practical significance is that prediabetes is often reversible. Modest weight loss, around 5 to 7 percent of body weight, combined with regular physical activity has been shown to significantly reduce the risk of progressing to type 2 diabetes. Your provider may also order an A1C test, which reflects your average blood sugar over the past two to three months, to get a broader picture beyond a single morning snapshot.
If your fasting glucose comes back normal but you have risk factors like a family history of diabetes, obesity, or a history of gestational diabetes, periodic retesting every one to three years is typical. A single normal result is reassuring, but blood sugar regulation can change over time.