High blood glucose can result from dozens of different triggers, and many of them have nothing to do with eating too much sugar. A fasting reading above 100 mg/dL is considered elevated, while 126 mg/dL or higher on two separate tests meets the diagnostic threshold for diabetes. Understanding the full range of causes helps you figure out why your numbers may be climbing and what you can realistically do about it.
Insulin Resistance and How It Develops
Insulin is the hormone that moves glucose out of your bloodstream and into your cells for energy. When cells stop responding to insulin efficiently, glucose builds up in the blood. This is called insulin resistance, and it’s the most common driver of chronically high glucose levels. It develops gradually, often over years, and is closely tied to excess body fat (especially around the abdomen), physical inactivity, and genetics.
Insulin resistance is the core problem behind both prediabetes and type 2 diabetes. In prediabetes, fasting glucose sits between 100 and 125 mg/dL. The pancreas is still producing insulin, but cells aren’t using it well. Over time, the pancreas can’t keep up with the extra demand, and blood sugar rises further. In type 1 diabetes, the issue is different: the immune system destroys the cells that make insulin, so glucose has no way to enter cells at all.
Stress and the Cortisol Connection
When you’re under physical or emotional stress, your body releases cortisol and adrenaline. These hormones signal the liver to dump stored glucose into the bloodstream, a survival mechanism designed to fuel a quick physical response. Cortisol also blocks the ability of other tissues to absorb glucose, keeping blood sugar available for the brain and muscles. Research published in the American Journal of Physiology found that chronically elevated cortisol promotes glucose production in the liver while simultaneously making other cells less responsive to insulin.
This means ongoing stress, whether from a demanding job, chronic pain, illness, or anxiety, can keep glucose levels elevated even if your diet hasn’t changed. Surgery, infections, and injuries trigger the same hormonal cascade, which is why blood sugar often spikes during hospital stays or recovery from illness.
Medications That Raise Blood Sugar
Corticosteroids are the single most common drug-related cause of high glucose. Medications like prednisone, prescribed for conditions like asthma, rheumatoid arthritis, and inflammatory bowel disease, directly increase liver glucose output and reduce insulin sensitivity. A meta-analysis of 12 studies found that roughly 18.6% of people on systemic corticosteroids developed drug-induced diabetes. At higher doses (25 mg or more of prednisone daily), the one-year risk of developing diabetes jumped to 5%, compared to just 0.9% in people not taking the drug.
Even low doses carry measurable risk. People taking less than 5 mg daily still had about twice the diabetes risk of non-users. The effect is dose-dependent, meaning higher doses and longer courses raise glucose more. Other medications known to push glucose up include certain blood pressure drugs (thiazide diuretics, beta-blockers), some antipsychotics, and immunosuppressants used after organ transplants.
Sleep and Blood Sugar
Poor sleep has a surprisingly strong effect on glucose regulation. A study from the University of Chicago found that after just four nights of restricted sleep, total-body insulin response dropped by an average of 16%, and fat cells became 30% less sensitive to insulin. That’s a significant shift from a short period of poor rest.
Sleep apnea compounds the problem. Repeated drops in oxygen overnight trigger stress hormones and inflammation, both of which worsen insulin resistance. If your fasting glucose is creeping up and you snore heavily or wake up feeling unrested, sleep quality is worth investigating as a contributing factor.
Why Morning Readings Are Often High
Many people are puzzled to find their blood sugar highest first thing in the morning, before they’ve eaten anything. Two distinct mechanisms explain this.
The dawn phenomenon is the more common one. Between roughly 3 and 8 a.m., your body releases cortisol and growth hormone to prepare you for waking. These hormones tell the liver to release glucose. In people without diabetes, the pancreas simply produces more insulin to compensate. If you have diabetes or prediabetes, that compensatory insulin response falls short, and you wake up with elevated numbers.
The Somogyi effect is less common and works differently. If your blood sugar drops too low overnight, perhaps because you skipped dinner or took too much insulin in the evening, your body overcorrects by flooding the bloodstream with glucose. The result looks the same on your morning meter reading, but the cause is the opposite: low blood sugar triggering a rebound high. Checking your glucose at 2 or 3 a.m. for a few nights can help distinguish between the two.
Hormonal Conditions
Several hormonal disorders directly raise blood glucose. Polycystic ovary syndrome (PCOS) is one of the most widespread. The condition involves insulin resistance as a core feature, not just a side effect. More than half of women with PCOS develop type 2 diabetes by age 40, according to the CDC.
Cushing’s syndrome, which involves chronically elevated cortisol (from a tumor or long-term steroid use), raises glucose through the same liver mechanisms described above. Hyperthyroidism speeds up metabolism in ways that can increase glucose absorption from the gut and accelerate the liver’s glucose output. Acromegaly, caused by excess growth hormone, also worsens insulin resistance. These conditions are less common than PCOS but worth considering if glucose levels are high without an obvious lifestyle explanation.
Diet, Dehydration, and Other Daily Factors
The most straightforward cause of a glucose spike is eating more carbohydrates than your body can process efficiently. Refined carbs and sugary drinks cause rapid spikes because they’re absorbed quickly. But even whole grains, fruit, and starchy vegetables raise glucose; they just do it more slowly. Portion size matters as much as food quality. A large bowl of brown rice will raise blood sugar more than a small one, regardless of how “healthy” it is.
Dehydration concentrates glucose in a smaller volume of blood, so your reading can appear higher simply because you haven’t had enough water. The CDC lists dehydration as one of the surprising factors that spikes blood sugar. This is especially relevant for morning readings if you tend to drink little fluid in the evening.
Physical inactivity matters on a daily basis, not just over months or years. Your muscles are the largest consumers of blood glucose, and they absorb it most efficiently during and immediately after movement. A sedentary day, particularly after a large meal, gives glucose nowhere to go. Even a 15-minute walk after eating can meaningfully blunt a post-meal spike.
Infections and Illness
Any kind of illness, from a common cold to a urinary tract infection, triggers an inflammatory and hormonal stress response that raises blood glucose. The body prioritizes keeping fuel available for the immune system, so the liver increases glucose output while cells become temporarily more insulin resistant. People with diabetes often notice their numbers are hardest to control when they’re sick, even if they’re eating less than usual. This is sometimes called “sick day” hyperglycemia, and it can persist for days after other symptoms resolve.