Regular (normal) blood sugar falls below 100 mg/dL when measured after fasting, and below 140 mg/dL when measured two hours after eating. These two numbers are the benchmarks doctors use to distinguish healthy blood sugar from the early stages of diabetes. But blood sugar isn’t static. It shifts throughout the day in response to food, movement, stress, and sleep, all managed by a network of hormones working behind the scenes.
Normal Ranges at a Glance
Blood sugar is measured in milligrams per deciliter (mg/dL) in the United States. The key numbers break into three categories:
- Fasting (no food for 8+ hours): Below 100 mg/dL is normal. Between 100 and 125 mg/dL signals prediabetes. At 126 mg/dL or higher, it crosses into diabetic range.
- Two hours after eating: Below 140 mg/dL is normal for people without diabetes.
- A1C (a 2- to 3-month average): Below 5.7% is normal. Between 5.7% and 6.4% indicates prediabetes. At 6.5% or higher, diabetes is diagnosed.
Fasting blood sugar and A1C are the two most common screening tests. The fasting test captures a single snapshot, while A1C reflects your average blood sugar over roughly three months. Both need to be in the abnormal range (often confirmed with a repeat test) before a diabetes diagnosis is made.
Why Blood Sugar Rises and Falls All Day
Your blood sugar doesn’t sit at one number. It’s constantly moving. After you eat, glucose from digested food enters your bloodstream and your levels climb. Between meals and overnight, levels drift back down. In a healthy person, blood sugar typically stays within a fairly narrow band because the body has a sophisticated system of hormones keeping it in check.
Even while you sleep, blood sugar rises as part of your natural circadian rhythm. This overnight increase is normal and not a sign of a problem in healthy people. It’s driven by hormonal signals that prepare your body for waking up.
How Your Body Keeps Blood Sugar Stable
Two hormones from the pancreas do most of the heavy lifting. Insulin, produced by beta cells, is released continuously into the bloodstream and surges after meals. It acts like a key, unlocking muscle, fat, and liver cells so they can absorb glucose and store it for later. When insulin levels are high, blood sugar drops.
Glucagon works in the opposite direction. Released by alpha cells in the pancreas, it signals the liver to break down its stored starch (glycogen) and release glucose back into the blood. Glucagon is most active overnight and between meals, preventing your blood sugar from dropping too low while you’re not eating.
Several other hormones fine-tune this system. When food hits your gut, it triggers the release of hormones called incretins, which tell beta cells to ramp up insulin production while simultaneously dialing down glucagon. Another hormone released alongside insulin slows stomach emptying and signals fullness to the brain, helping prevent a sharp post-meal spike.
Stress hormones add another layer. Adrenaline, cortisol, and growth hormone all raise blood sugar. Adrenaline acts directly on the liver to push glucose out quickly, which is useful during a fight-or-flight response. Cortisol makes muscle and fat cells less responsive to insulin, keeping more glucose available in the blood. Growth hormone has a similar insulin-blocking effect. This is why periods of physical or emotional stress can temporarily push blood sugar higher, even in people without diabetes.
What Pushes Blood Sugar Outside Normal Range
Exercise, stress, illness, sleep quality, and the composition of your meals all create fluctuations. A high-carbohydrate meal produces a bigger glucose spike than a meal built around protein, fat, and fiber. Physical activity pulls glucose into muscles for energy, which can lower blood sugar for hours afterward. Poor or short sleep disrupts the hormonal balance that keeps glucose in check, and chronic stress keeps cortisol elevated, which nudges fasting levels upward over time.
These short-term shifts are normal. The concern starts when fasting levels consistently land between 100 and 125 mg/dL, or when A1C drifts into the 5.7% to 6.4% range. That territory is prediabetes, meaning the body’s insulin system is struggling to keep up. About 1 in 3 American adults falls into this category, often without knowing it, because prediabetes rarely causes noticeable symptoms.
When Blood Sugar Drops Too Low
A blood sugar level at or below 70 mg/dL is considered low (hypoglycemia). Symptoms include shakiness, sweating, confusion, irritability, and a rapid heartbeat. In people without diabetes, this most often happens after skipping meals, intense exercise, or drinking alcohol on an empty stomach. The body usually corrects it quickly through glucagon release, but eating or drinking something with fast-acting carbohydrates (juice, glucose tablets, candy) speeds recovery.
Hypoglycemia is more common and more dangerous in people taking insulin or certain diabetes medications, because those drugs can push blood sugar lower than the body’s natural feedback system would allow.
Blood Sugar During Pregnancy
Pregnancy naturally changes how the body handles glucose. For women who already have type 1 or type 2 diabetes, targets are tighter than the general population: a fasting goal of 70 to 95 mg/dL and a post-meal target of 100 to 120 mg/dL or lower. A1C targets during pregnancy generally fall below 6.0% to 7.0%, depending on individual risk.
Gestational diabetes, which develops during pregnancy in women who didn’t previously have diabetes, is screened for between weeks 24 and 28 with a glucose tolerance test. After delivery, blood sugar usually returns to normal, but the experience signals a higher lifetime risk of developing type 2 diabetes later.
What A1C Tells You That a Single Test Can’t
A fasting blood sugar test captures one moment. You could test normal in the morning after a good night’s sleep and a light dinner, yet still have blood sugar that spikes too high after meals or runs elevated most of the day. A1C fills that gap by measuring how much glucose has attached to your red blood cells over their roughly three-month lifespan. It’s a rolling average, not a snapshot.
An A1C below 5.7% corresponds to an average blood sugar around 117 mg/dL or less. At 6.5%, the average is closer to 140 mg/dL. The test doesn’t require fasting, which makes it convenient, but it can be less accurate in people with certain blood disorders or recent blood loss that affects red blood cell turnover.