High blood glucose happens when your body can’t move sugar from your bloodstream into your cells efficiently. A fasting level of 100 mg/dL or above is considered elevated, with 126 mg/dL or higher pointing to diabetes. The causes range from what you ate for dinner to chronic conditions like insulin resistance, and understanding which factors are at play helps you figure out what to do about it.
How Your Body Normally Controls Blood Sugar
Every time you eat, your digestive system breaks carbohydrates down into glucose, which enters your bloodstream. Your pancreas responds by releasing insulin, a hormone that acts like a key, unlocking your cells so they can absorb that glucose for energy. Your liver also plays a central role: it stores extra glucose as glycogen and releases it back into your blood between meals to keep levels steady.
High blood glucose occurs when something disrupts this system. Either your pancreas isn’t producing enough insulin, your cells have stopped responding to insulin properly (insulin resistance), or your liver is dumping too much stored glucose into your bloodstream. Most causes of high glucose trace back to one or more of these three breakdowns.
Insulin Resistance and Type 2 Diabetes
The most common reason for persistently high glucose is insulin resistance. Your pancreas is still making insulin, but your muscles, fat, and liver don’t respond to it the way they should. The pancreas compensates by producing more and more insulin, but eventually it can’t keep up. That’s when blood sugar starts climbing.
Insulin resistance develops gradually, often over years. Excess body fat, especially around the midsection, is a major driver. So is physical inactivity. Genetics play a role too: if a parent or sibling has type 2 diabetes, your risk is significantly higher. By the time someone is diagnosed with type 2 diabetes (fasting glucose at or above 126 mg/dL, or an A1C of 6.5% or higher), insulin resistance has typically been building for a long time. Prediabetes, where fasting glucose sits between 100 and 125 mg/dL, is the warning stage.
What You Eat and How It Hits Your Blood Sugar
Not all foods raise blood sugar equally. Refined carbohydrates like white bread, sugary drinks, and white rice break down quickly and cause a sharp spike. Foods with a low glycemic index, such as whole grains, legumes, and most vegetables, are digested more slowly and produce a gentler rise. An international scientific consensus found that diets low in glycemic index and glycemic load are particularly important for people with insulin resistance, both for preventing and managing diabetes and heart disease.
Portion size matters as much as food type. A small serving of white rice will raise your glucose less than a massive bowl of brown rice. That’s the difference between glycemic index (how fast a food raises blood sugar) and glycemic load (how much it raises blood sugar based on both speed and quantity). Paying attention to both gives you a more accurate picture of what a meal will do to your levels.
Stress Hormones and the Liver
When you’re stressed, whether from a work deadline, an argument, or a physical injury, your body releases cortisol and adrenaline. These hormones signal your liver to convert its stored glycogen into glucose and release it into your bloodstream. This is a survival mechanism: your body is preparing for a fight-or-flight response and wants quick energy available.
The problem is that chronic stress keeps this system activated. Cortisol also makes your cells less responsive to insulin, so not only is more glucose being pumped out, but less of it is being absorbed. For someone already dealing with insulin resistance, ongoing stress can push glucose levels noticeably higher.
Sleep Deprivation
Poor sleep has a surprisingly direct effect on blood sugar. A study published in the journal Diabetes found that just one week of restricted sleep reduced insulin sensitivity by about 20% in healthy men. Ninety percent of the subjects in the study saw their insulin sensitivity drop. A separate measurement in the same study confirmed an 11% reduction using a different testing method. These weren’t people with diabetes. They were healthy volunteers whose bodies simply couldn’t handle glucose as well after sleeping less.
Shift work, sleep apnea, and even staying up late on weekends can disrupt your circadian rhythm enough to affect how your body processes sugar the next day. If your fasting glucose is creeping up and you’re regularly getting fewer than seven hours of sleep, the two may be connected.
Illness and Infection
Getting sick almost always raises blood sugar, even in people without diabetes. When your immune system detects an infection, it releases hormones that temporarily increase glucose levels. Your body does this intentionally: immune cells need extra fuel to fight off the invader. The trade-off is that these same hormones reduce insulin’s effectiveness, so glucose builds up in the bloodstream.
This is why people with diabetes are often told to monitor their blood sugar more closely when they have a cold, the flu, or a urinary tract infection. Glucose can spike significantly during illness and may take days to return to baseline after symptoms resolve.
Medications That Raise Blood Sugar
Several common medications can push glucose higher as a side effect. Corticosteroids (like prednisone) are the most well-known culprit. Over one-third of patients treated with these drugs develop elevated blood sugar. Steroids work against insulin on multiple fronts: they cause the liver to produce more glucose, they make muscle and fat cells more resistant to insulin, and with prolonged use, they can damage the insulin-producing cells in the pancreas. This creates a cycle where the body is both overproducing glucose and losing its ability to clear it.
Antiretroviral medications used to treat HIV can also disrupt glucose metabolism by affecting how cells generate energy, sometimes leading to a condition called antiretroviral-associated diabetes. Other medications known to raise blood sugar include certain blood pressure drugs (thiazide diuretics, beta-blockers), some antipsychotic medications, and immunosuppressants used after organ transplants. If you’ve noticed your glucose climbing after starting a new medication, the drug itself may be the cause.
High Morning Glucose
Waking up with high blood sugar is a common frustration, and two different mechanisms can explain it. The dawn phenomenon occurs because your body naturally releases cortisol and growth hormone in the early morning hours, typically between 4 and 8 a.m. These hormones tell your liver to release glucose so you have energy to start the day. In people with insulin resistance, there isn’t enough insulin activity to counteract this release, so glucose is elevated by the time you check it.
The Somogyi effect is less common and works differently. If blood sugar drops too low during the night (often from too much insulin before bed), the body treats it as an emergency. It releases adrenaline, cortisol, growth hormone, and glucagon, all of which tell the liver to dump stored glucose into the bloodstream. The result is a rebound high that shows up on your morning reading. Checking your blood sugar at 2 or 3 a.m. can help distinguish between the two: if it’s low at that hour, the Somogyi effect is more likely. If it’s normal or already rising, the dawn phenomenon is the cause.
Other Medical Conditions
Diabetes isn’t the only condition that disrupts glucose regulation. Polycystic ovary syndrome (PCOS) is closely linked to insulin resistance and often causes elevated glucose, sometimes years before a diabetes diagnosis. Cushing’s syndrome, where the body produces too much cortisol, raises blood sugar through the same stress-hormone pathway described above, just chronically. Pancreatitis and pancreatic damage can reduce insulin production directly by destroying the cells that make it.
Hormonal conditions involving the thyroid or pituitary gland can also interfere with glucose metabolism. Hyperthyroidism speeds up digestion and can cause glucose to enter the bloodstream faster than insulin can handle. Growth hormone excess (acromegaly) promotes insulin resistance. These conditions are less common, but worth considering if glucose is elevated and diabetes doesn’t seem to fit the picture.
Diagnostic Thresholds to Know
Understanding where your numbers fall helps you interpret what’s happening. The American Diabetes Association defines the ranges as follows:
- Normal fasting glucose: below 100 mg/dL
- Prediabetes: fasting glucose of 100 to 125 mg/dL, or an A1C of 5.7% to 6.4%
- Diabetes: fasting glucose of 126 mg/dL or higher, an A1C of 6.5% or higher, or a random glucose reading of 200 mg/dL or higher with symptoms
An oral glucose tolerance test, which measures blood sugar two hours after drinking a sugary solution, diagnoses prediabetes at 140 to 199 mg/dL and diabetes at 200 mg/dL or above. A single high reading doesn’t necessarily mean diabetes. Testing is typically repeated on a different day to confirm the result.