A normal fasting glucose level is below 100 mg/dL (5.6 mmol/L). Readings between 100 and 125 mg/dL fall into the prediabetes range, and a result of 126 mg/dL or higher on two separate tests indicates diabetes. These thresholds apply to both adults and children.
The Three Fasting Glucose Categories
Fasting glucose is measured after you haven’t eaten for at least 8 hours, typically first thing in the morning. The result places you into one of three categories:
- Normal: Below 100 mg/dL (5.6 mmol/L)
- Prediabetes: 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 tests
That second test matters. A single elevated reading can reflect a bad night of sleep, acute stress, or a testing error. Diabetes is only diagnosed when a high result is confirmed on a separate occasion. On the low end, a fasting glucose below 70 mg/dL is considered low blood sugar, and anything below 54 mg/dL is classified as severe hypoglycemia by the CDC.
Fasting Glucose During Pregnancy
Pregnant individuals are held to a tighter standard. For gestational diabetes screening and management, both ACOG and the ADA recommend a fasting glucose target below 95 mg/dL, not the usual 100 mg/dL cutoff. After meals, the target is below 140 mg/dL at one hour or below 120 mg/dL at two hours, measured from the start of eating. If you’re being monitored for gestational diabetes, your care team will likely ask you to track these numbers multiple times a day.
How to Prepare for the Test
You need to fast for 8 to 12 hours before the blood draw. During that window, plain water is fine and actually encouraged, since dehydration can affect results. But coffee, juice, soda, flavored water, and anything with calories or artificial sweeteners should be avoided. Alcohol may need to be skipped for a longer window beforehand, so check with whoever ordered the test.
Most people schedule fasting glucose tests for the morning, eating dinner the night before and skipping breakfast. If your test is at 8 a.m. and you stopped eating at 10 p.m., you’re well within the fasting window.
Why Your Morning Reading Might Be Higher Than Expected
It’s common for people, especially those with diabetes, to wake up with a fasting glucose that seems too high despite not eating for hours. There are a few reasons this happens.
The most common is the dawn phenomenon. In the early morning hours, your body releases cortisol and growth hormone, which signal the liver to push more glucose into the bloodstream. This is a normal part of waking up. In a healthy metabolism, the pancreas releases enough insulin to keep things balanced. But if you have diabetes or insulin resistance, that compensating insulin response falls short, and your fasting number climbs.
A second cause is waning insulin. If you take long-acting insulin and the dose is too low, or you injected it too early in the evening, it may not last through the night. Insulin pump users can see the same issue if their overnight basal rate is set too low.
A third, rarer cause is the Somogyi effect. If your blood sugar drops too low overnight, perhaps because you skipped dinner or took too much insulin with your evening meal, your body overcompensates by dumping glucose from the liver. You wake up with a high reading that’s actually a rebound from a low you slept through. Finally, a large dinner or bedtime snack can simply keep blood sugar elevated all night long.
Sleep and Stress Can Skew Your Numbers
Even if you don’t have diabetes, a rough night can raise your fasting glucose. Sleep deprivation activates your sympathetic nervous system, the “fight or flight” branch, which signals the liver to release more glucose. Research from Stanford’s Lifestyle Medicine program notes that poor sleep also promotes insulin resistance, meaning your cells respond less effectively to insulin even when it’s present. Sustained high cortisol from chronic stress compounds this by driving up circulating insulin and promoting fat storage around the midsection, both of which nudge fasting glucose upward over time.
If your reading comes back borderline, it’s worth considering whether you were particularly sleep-deprived or stressed before the test. A repeat test under better conditions can give a more accurate picture.
Home Monitors vs. Lab Draws
If you’re comparing a finger-stick reading from a home glucose meter to a lab result, expect some disagreement. Home meters measure glucose in whole blood, while labs measure it in plasma (the liquid portion of blood after cells are removed). Plasma readings run about 10 to 15% higher than whole blood readings. Most modern home meters automatically convert their results to plasma-equivalent values using a standard factor of 1.11, but not all do, and the conversion introduces a small margin of variability. For screening and diagnosis, a venous lab draw is the standard.
Fasting Glucose vs. A1c
Your fasting glucose is a snapshot of one morning. Your A1c (hemoglobin A1c) reflects your average blood sugar over roughly three months. The two tests don’t always tell the same story. Data from the National Glycohemoglobin Standardization Program shows that fasting glucose correlates less well with A1c than after-meal or bedtime glucose readings. As A1c rises, fasting glucose tends to underestimate the true average. This is why many providers order both tests: fasting glucose catches the overnight baseline, while A1c captures what’s happening across the full day, including the post-meal spikes you’d never see on a morning test.
For reference, a normal A1c is below 5.7%. Between 5.7% and 6.4% indicates prediabetes, and 6.5% or higher indicates diabetes. These thresholds apply to both adults and children.
What a Prediabetes Reading Actually Means
A fasting glucose of 100 to 125 mg/dL doesn’t mean you’re destined to develop diabetes. It means your body is starting to have trouble managing blood sugar efficiently. Insulin resistance is usually the underlying issue: your pancreas is still producing insulin, but your cells aren’t responding to it as well as they should, so glucose lingers in the bloodstream longer than normal.
The practical takeaway is that prediabetes responds well to lifestyle changes. Modest weight loss (even 5 to 7% of body weight), regular physical activity, improved sleep, and dietary adjustments that reduce refined carbohydrates can bring fasting glucose back below 100 mg/dL for many people. Prediabetes is the stage where intervention has the most impact, which is exactly why the threshold exists as a distinct category rather than lumping everything below 126 as “normal.”