A fasting blood sugar test should be taken first thing in the morning, after going 8 to 12 hours without eating or drinking anything other than plain water. Most people schedule the test early so the overnight hours cover most of the fasting window. Here’s what you need to know to get an accurate result.
How Long to Fast Before the Test
The American Diabetes Association defines fasting as no caloric intake for at least 8 hours. Most labs and doctors recommend a window of 8 to 12 hours. The simplest approach is to finish dinner by 8 or 9 p.m. and have your blood drawn the next morning. This way, you’re asleep for the majority of the fast.
During the fasting window, plain water is fine and even encouraged, since dehydration can make it harder for the technician to draw blood. But skip everything else: no coffee (even black), no tea, no flavored water, no gum, and no mints. Coffee contains compounds that can shift your blood sugar and metabolism enough to skew results. Stick to plain water only.
Why Morning Testing Matters
Your body’s hormones follow a daily rhythm. In the early morning hours, your liver releases stored glucose and your body produces hormones like cortisol and growth hormone that naturally raise blood sugar. This is called the dawn phenomenon, and it happens in everyone to some degree. In people with diabetes or prediabetes, the effect is more pronounced because their insulin response can’t fully compensate.
Because standardized diagnostic thresholds are based on morning readings taken after an overnight fast, testing at another time of day could give you a number that doesn’t match the ranges your doctor uses. A fasting test done in the afternoon, even after 8 hours without food, may not reflect the same metabolic picture.
What Your Numbers Mean
The 2025 American Diabetes Association standards break fasting blood sugar into three categories:
- Normal: below 100 mg/dL
- Prediabetes: 100 to 125 mg/dL
- Diabetes: 126 mg/dL or higher
A single reading at or above 126 mg/dL doesn’t automatically mean a diabetes diagnosis. Your doctor will typically confirm with a second test on a different day, or pair it with an A1C test, which reflects your average blood sugar over the past two to three months.
What Can Throw Off Your Results
Several things beyond food can push your fasting number higher or lower than it should be.
Your dinner the night before plays a bigger role than most people realize. Research shows that the glucose response to your last evening meal is strongly correlated with your fasting number the next morning, and that the carbohydrate content of that meal is a key driver. A heavy pasta dinner or a late-night snack loaded with refined carbs can elevate your morning reading. Eating a balanced, moderate dinner and avoiding late-night eating gives a cleaner result.
Certain medications are well-known to raise fasting glucose. Steroids (like prednisone) are the most common culprit, but blood pressure medications like metoprolol and atenolol, thiazide diuretics, statins, and some antipsychotic medications can all push fasting numbers up. If you take any of these, your doctor should already be aware, but it’s worth mentioning before the test so they can interpret the result in context.
Smoking can also affect blood sugar readings. Avoid cigarettes the morning of your test. Alcohol consumed the night before can cause unpredictable swings in blood sugar overnight, so it’s best to skip it entirely the evening before a fasting test.
Fasting Tests During Pregnancy
Most pregnant women are screened for gestational diabetes between 24 and 28 weeks. The test involves fasting for 8 to 14 hours, then having blood drawn before and after drinking a glucose solution. The fasting threshold is stricter during pregnancy: above 95 mg/dL is considered abnormal in the standard two-step test, and above 92 mg/dL in the one-step version. Your OB will tell you which version your lab uses.
If you had gestational diabetes, guidelines recommend a follow-up glucose test no later than 12 weeks after delivery, and then every 3 years going forward, since gestational diabetes significantly raises the long-term risk of type 2 diabetes.
How Often You Need This Test
For routine screening in adults without symptoms, most guidelines recommend starting at age 35, or earlier if you have risk factors like obesity, a family history of diabetes, or a history of gestational diabetes. If your results come back normal, retesting every 3 years is typical.
If you’ve been diagnosed with prediabetes, your doctor will likely test more frequently, often annually, to monitor whether your numbers are trending toward diabetes or improving with lifestyle changes. For people already diagnosed with diabetes, the A1C test (recommended at least twice a year) largely replaces routine fasting glucose tests for ongoing monitoring, though your doctor may still order fasting glucose periodically.
Morning Readings at Home
If you monitor blood sugar at home with a glucose meter, the same timing principles apply. Test immediately after waking, before eating, drinking coffee, or brushing your teeth (some toothpastes contain sweeteners that can affect readings). Keep your meter and strips at room temperature for accuracy.
Consistently high morning readings despite good control the rest of the day could signal the dawn phenomenon, which is common in people with type 2 diabetes. Less commonly, it could reflect what’s known as the Somogyi effect, where too much insulin the night before causes blood sugar to crash overnight and then rebound. These two patterns look similar on a morning reading but have opposite causes, so tracking your numbers over time and sharing them with your doctor helps sort out what’s happening.