Blood glucose level is the concentration of sugar (glucose) circulating in your bloodstream at any given moment. Glucose is your body’s primary fuel source, derived from the food you eat. A normal fasting blood glucose level is below 100 mg/dL, and your body works constantly to keep it within a tight range through a hormonal balancing act between insulin and glucagon.
How Your Body Regulates Blood Sugar
When you eat, your digestive system breaks down carbohydrates, proteins, and fats. Most of that food eventually converts into glucose, which enters your bloodstream. As blood glucose rises, your pancreas releases insulin, a hormone that acts like a key, unlocking your cells so glucose can enter and be used for energy.
When blood sugar drops too low, a second hormone called glucagon kicks in. Glucagon signals your liver to convert stored glucose back into a usable form and release it into the bloodstream. It also tells the liver to stop absorbing glucose so more stays available. These two hormones, insulin pushing glucose down and glucagon pushing it back up, work as a continuous feedback loop that keeps your levels stable throughout the day.
Normal Blood Glucose Ranges
Blood sugar isn’t a single fixed number. It shifts throughout the day depending on when and what you ate, your activity level, and even your stress. But the ranges below represent what clinicians consider healthy:
- Fasting (no food for 8+ hours): Below 100 mg/dL (5.6 mmol/L)
- Two hours after eating: Below 140 mg/dL (7.8 mmol/L)
If your fasting glucose falls between 100 and 125 mg/dL, that’s considered prediabetes, a signal that your body is starting to struggle with blood sugar regulation. A fasting level of 126 mg/dL or higher, confirmed on a repeat test, meets the diagnostic threshold for diabetes.
Prediabetes and Diabetes Thresholds
Doctors use several tests to evaluate blood sugar, and each has its own cutoff points. The American Diabetes Association recognizes three main categories:
- Fasting plasma glucose: Normal is below 100 mg/dL. Prediabetes is 100 to 125 mg/dL. Diabetes is 126 mg/dL or higher.
- Oral glucose tolerance test (two hours after drinking a sugary solution): Normal is below 140 mg/dL. Prediabetes is 140 to 199 mg/dL. Diabetes is 200 mg/dL or higher.
- A1C (a measure of average blood sugar over roughly three months): Normal is below 5.7%. Prediabetes is 5.7% to 6.4%. Diabetes is 6.5% or higher.
A random blood glucose test, taken at any time regardless of meals, can also diagnose diabetes if it comes back at 200 mg/dL or higher and you have symptoms like excessive thirst or frequent urination.
What A1C Tells You About Daily Averages
While a fasting glucose test captures a single snapshot, the A1C test reflects your average blood sugar over the past two to three months. It measures the percentage of hemoglobin (a protein in red blood cells) that has glucose attached to it. The higher your blood sugar has been running, the more glucose sticks to hemoglobin.
The relationship between A1C and daily averages is roughly linear. An A1C of 5% corresponds to an estimated average glucose of about 97 mg/dL. At 6%, it’s around 126 mg/dL. At 7%, roughly 154 mg/dL. Each percentage point increase adds about 28 to 29 mg/dL to the daily average. So if your A1C comes back at 8%, your blood sugar has been averaging around 183 mg/dL, well into a range where sustained damage to blood vessels and nerves becomes a real concern.
When Blood Sugar Drops Too Low
Hypoglycemia, or low blood sugar, generally means a reading at or below 70 mg/dL. It’s most common in people taking insulin or certain diabetes medications, but it can happen to anyone under the right circumstances, like skipping meals or exercising intensely without eating.
Early symptoms tend to come on fast: shakiness, sweating, a racing heartbeat, sudden hunger, dizziness, and difficulty concentrating. You might feel anxious or irritable for no clear reason, or notice tingling in your lips or tongue. These are your body’s alarm signals, driven by a surge of stress hormones trying to push glucose back up.
If blood sugar continues to fall, symptoms become more serious: confusion, slurred speech, blurry vision, and loss of coordination. Severe hypoglycemia can cause seizures or loss of consciousness. Eating or drinking something with fast-acting sugar (juice, glucose tablets, regular soda) is the standard response for mild episodes.
When Blood Sugar Stays Too High
Hyperglycemia often produces no noticeable symptoms until blood sugar climbs above 180 to 200 mg/dL. At that point, you might notice increased thirst, frequent urination, fatigue, or blurred vision. Because it can be silent for long stretches, chronically elevated blood sugar sometimes causes damage before a person realizes anything is wrong.
The long-term consequences of untreated high blood sugar are significant. Sustained hyperglycemia damages blood vessels throughout the body, leading to complications that include cardiovascular disease, nerve damage (especially in the feet and hands), kidney damage that can progress to kidney failure, and retinal damage that can cause blindness. Poor circulation and nerve damage in the feet can lead to slow-healing wounds and, in severe cases, amputation. Even gum disease and joint problems become more common.
Surprising Factors That Affect Your Levels
Food is the most obvious driver of blood sugar changes, but it’s far from the only one. Poor sleep, even a single night of it, reduces your body’s ability to use insulin effectively the next day. Skipping breakfast can paradoxically raise blood sugar after lunch and dinner. Dehydration concentrates the glucose already in your blood, pushing readings higher without you having eaten a thing.
Stress is a major factor. Physical stress like a sunburn or illness and psychological stress both trigger hormone releases that raise blood sugar. Caffeine affects some people’s glucose levels even without added sugar. The time of day matters too: blood sugar tends to be harder to control later in the day, and most people experience a natural hormone surge in the early morning hours (sometimes called the dawn phenomenon) that temporarily elevates glucose levels.
Extreme heat can have the opposite effect, causing blood vessels to widen and speeding up insulin absorption, which may lead to a blood sugar drop. Even certain nasal decongestant sprays contain chemicals that prompt the liver to release extra glucose.
How Blood Sugar Is Monitored
The traditional method is a finger-stick test using a portable glucose meter. You prick your finger, place a drop of blood on a test strip, and the meter displays your current blood sugar within seconds. This approach gives you an accurate reading at that exact moment but tells you nothing about what happened between tests.
Continuous glucose monitors (CGMs) take a different approach. A small sensor inserted just under the skin reads glucose levels in the fluid surrounding your cells, typically every few minutes, and transmits the data to a phone or receiver. This creates a detailed picture of how your blood sugar moves throughout the day and night, including spikes and dips you’d otherwise miss entirely.
In a 12-month clinical trial comparing the two methods in people with insulin-treated type 2 diabetes, CGM users spent significantly more time in their target glucose range (a 15.2 percentage point improvement over finger-stick users). They also saw larger reductions in A1C (an additional 0.9 percentage point drop), needed less daily insulin, and reported better overall well-being and satisfaction with their diabetes management. CGM is increasingly used not just for diabetes management but also by people with prediabetes who want to understand how their body responds to specific foods and habits.