High blood sugar happens when glucose builds up in your bloodstream faster than your body can move it into cells. A fasting blood sugar above 100 mg/dL signals that something is off, and 126 mg/dL or higher on two separate tests meets the threshold for diabetes. The causes range from what you ate for lunch to how well you slept last night, and understanding them can help you figure out which factors are driving your numbers up.
How Your Body Normally Controls Blood Sugar
When you eat carbohydrates, your digestive system breaks them into glucose, which enters your bloodstream. Your pancreas responds by releasing insulin, a hormone that acts like a key, unlocking cells in your muscles, liver, and fat tissue so they can absorb that glucose for energy. Between meals, your liver releases small amounts of stored glucose to keep levels steady.
High blood sugar occurs when this system breaks down at any point: your pancreas doesn’t make enough insulin, your cells stop responding to it properly (insulin resistance), or your liver dumps too much glucose. Most causes of high blood sugar trace back to one or more of these three problems.
Insulin Resistance and Type 2 Diabetes
The most common reason for persistently high blood sugar is insulin resistance, the hallmark of type 2 diabetes. Your pancreas still produces insulin, but your muscles, liver, and fat cells don’t respond to it efficiently. The pancreas compensates by producing more, but over time it can’t keep up, and blood sugar climbs. Excess body fat, particularly around the abdomen, is one of the strongest drivers of insulin resistance, though it also develops in people at a healthy weight due to genetics, aging, or other metabolic factors.
Prediabetes sits between normal and diabetic ranges. That means a fasting blood sugar of 100 to 125 mg/dL or an A1C between 5.7% and 6.4%. At this stage, insulin resistance is already present but hasn’t yet overwhelmed the pancreas completely.
Autoimmune Destruction in Type 1 Diabetes
In type 1 diabetes, the immune system attacks and destroys the insulin-producing cells in the pancreas. Without those cells, the body produces little to no insulin, and blood sugar rises dramatically. The triggers for this autoimmune response involve both genetic predisposition and environmental factors. Viral infections have been associated with triggering the immune attack, and certain genes related to insulin production influence a person’s susceptibility. Unlike type 2 diabetes, type 1 is not caused by diet or lifestyle and typically appears in childhood or young adulthood, though it can develop at any age.
What You Eat (and How Much)
Carbohydrates have the most direct effect on blood sugar. But not all carbs hit your bloodstream at the same speed. The glycemic index ranks foods on a scale of 0 to 100 based on how quickly they raise blood sugar, with pure glucose at 100. Processed foods generally score higher, while foods rich in fiber or fat score lower because they slow digestion and glucose absorption.
The glycemic index only tells part of the story, though. Watermelon has a high glycemic index of 80, but a typical serving contains so little carbohydrate that its real-world impact on blood sugar is modest. A separate measure called glycemic load accounts for both speed and quantity of carbohydrate in a serving, giving a more accurate picture of what actually happens after you eat. A bowl of white rice, for instance, delivers both fast-absorbing carbs and a large carbohydrate load, which is why it spikes blood sugar more than most fruits.
Portion size matters as much as food choice. Eating a large plate of even moderate-glycemic foods can overwhelm your insulin response, especially if insulin resistance is already present.
Stress and the Hormonal Surge
Physical or emotional stress triggers a cascade of hormonal changes designed to fuel a fight-or-flight response. Your body suppresses insulin release, ramps up glucose production in the liver, and reduces how effectively your cells absorb glucose. The result is a rapid rise in blood sugar, even if you haven’t eaten anything.
This response is driven by the sympathetic nervous system signaling directly to the liver, independent of the better-known stress hormone cortisol. Your brain essentially tells the liver to dump its glucose stores into the bloodstream, while simultaneously dialing down insulin so that glucose stays available for muscles and the brain. For someone without diabetes, the system rebalances once the stress passes. For someone with insulin resistance or diabetes, the rebalancing is slower and less complete, which means stress can cause prolonged high readings.
Sleep Deprivation
Losing even a modest amount of sleep measurably changes how your body handles glucose. A Columbia University study found that shortening sleep by just 90 minutes per night for six weeks increased insulin resistance by nearly 15% in women. Fasting insulin levels rose by over 12%, and the effect was even more pronounced in certain groups: postmenopausal women saw insulin resistance climb by more than 20%, and premenopausal women saw fasting insulin spike by over 15%.
Notably, the researchers found these effects were independent of changes in body fat. Sleep loss was directly impairing insulin-producing cells and metabolism on its own, not just through weight gain. This means that consistently short or poor-quality sleep can raise your blood sugar even if your diet and weight stay the same.
Illness and Infection
When your body fights an infection, the immune system releases inflammatory signaling molecules that interfere with insulin’s ability to work. This is why people with diabetes often see their blood sugar spike during a cold, flu, urinary tract infection, or any other illness. The inflammation promotes insulin resistance in the liver and fat tissue, and stress hormones released during illness compound the problem by driving additional glucose production. Even mild infections can push blood sugar noticeably higher for days.
Medications That Raise Blood Sugar
A surprisingly long list of common medications can push blood sugar up, sometimes significantly. The main culprits work by increasing insulin resistance, reducing insulin production, or both.
- Corticosteroids (like prednisone) are among the most potent offenders. They increase insulin resistance, boost glucose production in the liver, and decrease insulin output all at once.
- Atypical antipsychotics raise blood sugar through both weight gain and direct effects on insulin-producing cells.
- Thiazide diuretics, commonly prescribed for blood pressure, increase insulin resistance and inhibit glucose-triggered insulin release.
- Beta-blockers, another blood pressure medication class, block the initial burst of insulin your pancreas normally releases when blood sugar rises and reduce glucose uptake by muscles.
- Statins, widely used for cholesterol, are associated with decreased insulin sensitivity and reduced insulin secretion.
- Niacin (vitamin B3 supplements at high doses) increases glucose production in the liver while worsening insulin resistance.
If you take any of these and notice higher blood sugar readings, the medication may be a contributing factor worth discussing at your next appointment.
Why Blood Sugar Spikes in the Morning
Many people notice their highest readings first thing in the morning, before eating anything. Two distinct patterns explain this.
The dawn phenomenon is the more common one. In the early morning hours, your body naturally reduces insulin output while increasing hormones that raise blood sugar, particularly growth hormone, cortisol, and adrenaline. This hormonal shift is normal and happens in everyone, but in people with diabetes or prediabetes, there isn’t enough insulin to counteract it, so blood sugar climbs before you wake up.
The Somogyi effect looks similar on a morning glucose reading but has the opposite origin. It happens when blood sugar drops too low during the night (often from too much insulin or medication), and the body overcompensates by flooding the bloodstream with glucose. The way to tell them apart is to check blood sugar between 3 a.m. and 5 a.m. over several nights. If your reading at that hour is normal or high, it’s the dawn phenomenon. If it’s low, you’re likely experiencing a rebound from overnight hypoglycemia.
The Numbers That Define High Blood Sugar
The American Diabetes Association uses these thresholds:
- Normal fasting blood sugar: below 100 mg/dL
- Prediabetes: fasting 100 to 125 mg/dL, or A1C of 5.7% to 6.4%
- Diabetes: fasting 126 mg/dL or higher on two tests, A1C of 6.5% or higher, or a random reading of 200 mg/dL or higher with symptoms
After a glucose tolerance test (drinking a sugary solution and testing two hours later), normal is below 140 mg/dL, prediabetes falls between 140 and 199 mg/dL, and 200 mg/dL or above indicates diabetes. A single high reading doesn’t necessarily mean diabetes. Stress, illness, a recent meal, or a medication could be responsible. Persistent or repeated high readings are what matter for diagnosis.