Blood sugar levels measure the amount of glucose circulating in your bloodstream at any given moment. A normal fasting blood sugar is below 100 mg/dL, and levels shift throughout the day based on what you eat, how you sleep, and how active you are. Understanding these numbers helps you recognize whether your body is managing glucose well or showing early signs of a problem.
How Your Body Regulates Blood Sugar
Glucose is your body’s primary fuel source. When you eat carbohydrates, your digestive system breaks them down into glucose, which enters the bloodstream. From there, two hormones produced by the pancreas work as a balancing act to keep levels stable.
Insulin is released when blood sugar rises, typically after a meal. It acts like a key, unlocking cells in your muscles, fat, and liver so they can absorb glucose and use it for energy. As cells take in that glucose, blood sugar drops back toward its baseline. Glucagon does the opposite. When blood sugar falls too low, such as between meals or overnight, the pancreas releases glucagon. This hormone signals the liver to convert its stored glucose into a usable form and release it back into the bloodstream. Glucagon also prompts the body to produce glucose from other sources, like amino acids from protein. Together, insulin and glucagon counterbalance each other to keep blood sugar in a narrow, healthy range.
Normal Blood Sugar Ranges
Blood sugar isn’t a single fixed number. It fluctuates naturally throughout the day, and what counts as “normal” depends on when you last ate.
- Fasting (no food for 8+ hours): Below 100 mg/dL (5.6 mmol/L) is considered normal.
- Two hours after eating: Below 140 mg/dL is typical for someone without diabetes. Levels up to 180 mg/dL are used as a general target for people managing diabetes.
Your lowest readings usually occur after an overnight fast, while the highest come within the first one to two hours after a meal. Even in a perfectly healthy body, blood sugar can swing from the 70s to the 130s over the course of a day. The key is that it returns to baseline relatively quickly after eating.
A1C: The Bigger Picture
A single blood sugar reading is a snapshot. The A1C test provides something more like a three-month average. It measures the percentage of hemoglobin (a protein in red blood cells) that has glucose attached to it. Because red blood cells live for about three months, the test captures your overall glucose exposure during that window.
The diagnostic thresholds break down like this:
- Normal: Below 5.7%
- Prediabetes: 5.7% to 6.4%
- Diabetes: 6.5% or above
Each A1C percentage corresponds to an estimated average glucose. An A1C of 6% translates to an average blood sugar of about 126 mg/dL. At 7%, that average climbs to 154 mg/dL. By 9%, the estimated average is 212 mg/dL. The formula used to calculate this is straightforward: multiply the A1C by 28.7, then subtract 46.7. But in practice, your doctor handles that math and tells you where you stand.
What Low Blood Sugar Feels Like
Hypoglycemia generally refers to blood sugar dropping to 70 mg/dL or below. At this level, your body sends out alarm signals that are hard to ignore: shakiness, sweating, a sudden wave of hunger, dizziness, and a racing or irregular heartbeat. Some people feel anxious or irritable without an obvious cause, or notice tingling in their lips, tongue, or cheeks. Difficulty concentrating and fatigue are common too.
These symptoms happen because your brain depends heavily on glucose. When supply drops, it reacts fast. For most people, eating 15 to 20 grams of fast-acting carbohydrates (a few glucose tablets, half a cup of juice, or a tablespoon of honey) brings levels back up within minutes. Low blood sugar is most common in people who take insulin or certain diabetes medications, but it can also occur after prolonged fasting, intense exercise, or heavy alcohol consumption on an empty stomach.
What High Blood Sugar Feels Like
Hyperglycemia, or high blood sugar, often develops gradually enough that people don’t notice it right away. Early signs include increased thirst, frequent urination, headaches, and blurred vision. Your body is essentially trying to flush excess glucose out through your kidneys, which pulls extra water with it, leaving you dehydrated and thirsty in a cycle that feeds on itself.
When blood sugar stays elevated over weeks or months, symptoms shift. Fatigue becomes persistent. Wounds and cuts heal slowly. Skin infections or vaginal yeast infections may become recurring. Unexplained weight loss can occur because your cells aren’t absorbing glucose properly, forcing the body to burn fat and muscle for fuel instead.
The long-term consequences of chronically high blood sugar are serious. Excess glucose damages blood vessels and nerves throughout the body. Over years, this can lead to vision loss, kidney damage, nerve pain or numbness (especially in the feet and hands), digestive problems, heart disease, and stroke. These complications tend to be irreversible, which is why catching elevated blood sugar early, at the prediabetes stage, matters so much.
Factors That Shift Your Numbers
Food is the most obvious influence on blood sugar, but it’s far from the only one. Sleep has a surprisingly powerful effect. Even a single night of partial sleep deprivation increases insulin resistance, meaning your cells respond less effectively to insulin and more glucose stays in the bloodstream. Sleep loss also raises cortisol, a stress hormone that directly increases blood sugar. The timing of your sleep matters too: irregular sleep schedules can disrupt the normal daily rhythm of insulin and cortisol release.
Physical activity generally lowers blood sugar because working muscles pull glucose from the blood for energy. This effect can last for hours after exercise. Stress, whether physical or emotional, pushes blood sugar in the opposite direction. When you’re under stress, your body releases cortisol and adrenaline, which trigger the liver to dump stored glucose into the bloodstream as part of the fight-or-flight response. That’s useful if you’re running from danger, less useful if you’re sitting in traffic.
Illness and infection also raise blood sugar, sometimes dramatically, because the body’s inflammatory response increases insulin resistance. Even medications unrelated to diabetes, like steroids prescribed for allergies or joint pain, can cause temporary spikes.
Prediabetes: The Warning Window
Prediabetes means your blood sugar is higher than normal but hasn’t crossed the diabetes threshold. With a fasting glucose between 100 and 125 mg/dL, or an A1C between 5.7% and 6.4%, you’re in a range where the body’s glucose regulation is starting to falter. About 80% of people with prediabetes don’t know they have it because there are rarely noticeable symptoms at this stage.
The practical value of catching prediabetes is that it’s often reversible. Current nutrition guidance emphasizes incorporating plant-based protein and fiber into meals, limiting saturated fat to reduce cardiovascular risk, and choosing water over sugary or artificially sweetened drinks. Combined with regular physical activity and better sleep habits, these changes can bring blood sugar back into the normal range and significantly reduce the risk of progressing to type 2 diabetes.