Fasting for a blood sugar test means not eating or drinking anything except plain water for at least 8 hours before your blood is drawn. Most people schedule the test first thing in the morning so the overnight hours do most of the work. The fast gives your body time to finish processing your last meal, so the reading reflects your baseline blood sugar rather than a spike from recent food.
How Long You Need to Fast
The minimum is 8 hours, though some providers ask for 10 to 12 hours. If your test is at 8 a.m., stopping food and drinks (other than water) by midnight puts you comfortably in range. Your provider will tell you the exact window, but 8 hours is the standard cutoff used by labs across the country.
What You Can and Cannot Have
Plain water is the only thing allowed during the fast. Coffee, tea, juice, soda, and flavored water are all off-limits, even if they contain zero calories. Coffee in particular can affect glucose metabolism and blood pressure readings, which is why it’s excluded despite having no sugar on its own. Flavored sparkling water may contain sugars or artificial sweeteners that interfere with results, so skip those too.
The simplest rule: if it isn’t plain, unflavored water, don’t drink it until after the blood draw.
Why Fasting Matters for the Test
When you eat, your blood sugar rises as your body digests carbohydrates. It can stay elevated for several hours depending on the meal. A fasting test eliminates that variable. It measures how well your body manages blood sugar on its own, without any incoming food.
During an overnight fast, your liver keeps blood sugar from dropping too low. It breaks down stored glycogen into glucose and, when those stores run low, builds new glucose from amino acids and fat byproducts. A fasting blood sugar reading captures how effectively your body handles this process. If it’s too high even without food in the picture, that signals a problem with insulin production or insulin sensitivity.
What the Numbers Mean
Once your blood is drawn, the result falls into one of three categories based on thresholds set by the CDC and the American Diabetes Association:
- Normal: 99 mg/dL or below
- Prediabetes: 100 to 125 mg/dL
- Diabetes: 126 mg/dL or above
A single reading of 126 mg/dL or higher doesn’t automatically mean a diabetes diagnosis. Providers typically confirm the result with a second fasting test on a different day, or with an A1C test (which reflects your average blood sugar over the past two to three months). For the A1C, normal is below 5.7%, prediabetes is 5.7% to 6.4%, and diabetes is 6.5% or above. As of the 2024 ADA guidelines, these diagnostic thresholds remain unchanged.
Things That Can Throw Off Your Results
Even with a proper fast, a few factors can push your reading higher or lower than your true baseline.
The most common one is the dawn phenomenon. In the early morning hours, your body releases a surge of hormones including cortisol, growth hormone, and glucagon. These hormones naturally raise blood sugar to help you wake up, and they also make your cells slightly more resistant to insulin. For some people, especially those with diabetes or prediabetes, this surge produces a noticeably higher reading first thing in the morning compared to later in the day. If your fasting number seems unexpectedly high, this hormonal pattern may be part of the reason.
Exercise on the morning of the test can also skew results. Physical activity lowers blood sugar by pulling glucose into your muscles for energy. A brisk walk or workout before the draw could artificially lower your reading. Play it safe and skip exercise until after the test.
Stress and poor sleep both raise cortisol, which in turn raises blood sugar. A rough night before the test can nudge your number upward by a few points. This doesn’t mean you need perfect sleep, but it’s worth knowing that a single elevated result may not tell the full story.
Medications and Your Fast
If you take daily medications, especially for blood sugar, blood pressure, or thyroid conditions, ask your provider whether to take them before the test. Some medications need to be taken with food, which conflicts with the fast. Others, like insulin or certain diabetes pills, can lower blood sugar enough to affect the reading or make you feel unwell during the fast. Your provider may adjust the timing or dose for the morning of the test.
Don’t skip prescribed medications on your own. Just call ahead and ask for instructions specific to your situation.
How to Make the Fast Easier
Schedule the earliest available appointment so you spend less waking time without food. Eat a balanced dinner the night before with protein, fiber, and healthy fats, which digest slowly and help you stay comfortable longer. Keep a glass of water on your nightstand so you stay hydrated overnight. Dehydration can make the blood draw harder and leave you feeling lightheaded.
Bring a snack to eat right after the test. Most people feel fine after fasting, but your blood sugar will be at its lowest point of the day, and having something ready means you won’t have to wait.