A glucose level is the amount of sugar circulating in your bloodstream at any given moment, measured in milligrams per deciliter (mg/dL) in the United States or millimoles per liter (mmol/L) in most other countries. For a healthy adult who hasn’t eaten in at least eight hours, a normal reading falls below 100 mg/dL (5.6 mmol/L). This number shifts throughout the day based on what you eat, how active you are, and even how well you slept.
What the Numbers Mean
Your body keeps blood sugar in a surprisingly tight range. A healthy fasting glucose level sits between 70 and 99 mg/dL. Some people without diabetes can dip as low as 50 mg/dL without problems, but readings below 70 mg/dL are generally considered low blood sugar.
After eating, your blood sugar naturally rises. In a person without diabetes, it typically stays below 140 mg/dL two hours after a meal. For someone with diabetes, the target is below 180 mg/dL at that same two-hour mark. Blood sugar peaks roughly 60 to 90 minutes after eating and then gradually returns toward baseline as your body processes the sugar.
Normal, Prediabetes, and Diabetes Thresholds
Doctors use specific fasting glucose cutoffs to screen for blood sugar problems. These thresholds come from the American Diabetes Association:
- Normal: below 100 mg/dL (5.6 mmol/L)
- Prediabetes: 100 to 125 mg/dL (5.6 to 6.9 mmol/L)
- Diabetes: 126 mg/dL (7.0 mmol/L) or higher on two separate tests
Another common test, the A1C, measures your average blood sugar over the past two to three months. An A1C below 5.7% is normal, 5.7% to 6.4% indicates prediabetes, and 6.5% or higher points to diabetes. Fasting glucose gives you a snapshot of one moment, while A1C reveals the bigger picture.
How Your Body Controls Blood Sugar
Two hormones produced by the pancreas do most of the work. Insulin acts like a key, unlocking cells so they can absorb sugar from the blood and use it for energy. When blood sugar drops too low, a second hormone signals the liver to release stored sugar back into the bloodstream. These two hormones constantly adjust in response to meals, activity, and stress to keep glucose in a safe range. Additional hormones fine-tune the process, but insulin is by far the most important player.
In type 1 diabetes, the body stops making insulin entirely. In type 2 diabetes, cells gradually stop responding to insulin as effectively, and the pancreas can’t keep up with the increased demand. In both cases, blood sugar rises above normal thresholds.
Surprising Factors That Affect Your Levels
Food is the most obvious influence on blood sugar, but it’s far from the only one. The CDC lists several less obvious factors that can push glucose higher:
- Poor sleep: even one night of too little sleep can make your body use insulin less efficiently
- Stress and pain: physical stress, including something as simple as a sunburn, triggers hormones that raise blood sugar
- Caffeine: coffee without any sweetener can spike glucose in some people
- Dehydration: less water in your body means the sugar in your blood becomes more concentrated
- Skipping breakfast: going without a morning meal can lead to higher blood sugar after both lunch and dinner
- Time of day: 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 that temporarily raises glucose
Some nasal decongestant sprays contain chemicals that prompt the liver to release more sugar. Even gum disease has a two-way relationship with blood sugar, both worsening and being worsened by elevated glucose.
How Glucose Is Measured
There are three main ways to check blood sugar, each pulling from a slightly different source.
A standard lab blood draw takes a sample directly from a vein. This is the most accurate method and the one used for diagnosing diabetes. The fasting test requires at least eight hours without eating.
A finger-stick meter, the kind most people with diabetes use at home, tests a tiny drop of blood from the capillaries in your fingertip. These readings are close to lab values but not identical. They give a reliable picture of where your blood sugar stands right now.
Continuous glucose monitors (CGMs) measure sugar in the fluid between cells, not in the blood itself. This interstitial fluid lags behind blood sugar changes by anywhere from a few minutes to as long as 45 minutes. The gap widens when glucose is changing rapidly, such as right after a meal or during exercise. During physical activity, the difference between a CGM reading and a blood draw can be more than double what it is at rest. This is why a CGM might show a different number than a finger stick taken at the same moment, especially if your blood sugar is rising or falling quickly.
Low Blood Sugar: What to Watch For
Blood sugar below 70 mg/dL is considered low. Mild episodes can cause shakiness, sweating, irritability, and a racing heartbeat. These symptoms are your body’s alarm system, signaling you to eat or drink something with fast-acting sugar. A small glass of juice or a few glucose tablets is usually enough to bring levels back up within 15 minutes.
Severe low blood sugar is a medical emergency. It can cause confusion, loss of coordination, seizures, or loss of consciousness. People in this state typically need someone else to help them, because their thinking is too impaired to treat themselves. Severe episodes are most common in people taking insulin or certain diabetes medications, and they’re rare in people without diabetes.
High Blood Sugar Over Time
A single high reading after a big meal isn’t cause for alarm. Sustained high blood sugar is the problem. When glucose stays elevated for months or years, it damages blood vessels and nerves throughout the body. The kidneys, eyes, heart, and feet are especially vulnerable. This is why the A1C test matters so much: it reveals whether blood sugar has been consistently elevated, even if individual readings look fine on any given day.
Prediabetes is the stage where fasting glucose runs between 100 and 125 mg/dL. At this point, the damage hasn’t progressed far, and lifestyle changes like regular exercise, modest weight loss, and dietary shifts can often bring numbers back to a normal range. Not everyone with prediabetes progresses to diabetes, but without changes, many do.