Many things raise blood glucose, and most of them aren’t on your plate. Carbohydrates are the most obvious driver, but stress hormones, poor sleep, medications, intense exercise, dehydration, and even your body’s own internal clock all push glucose levels higher. Understanding each trigger helps you see the full picture of what’s happening in your body.
Carbohydrates: The Primary Driver
When you eat carbohydrates, your digestive system breaks them down into sugar that enters your bloodstream. As blood sugar rises, the pancreas releases insulin, which signals cells to absorb that sugar for energy or storage. Once cells take up the sugar, blood levels drop, and the pancreas switches to releasing glucagon, a hormone that tells the liver to release its stored sugar to keep levels from falling too low.
How fast this happens depends on the type of carbohydrate. Simple carbohydrates, found in white bread, candy, fruit juice, and sugary drinks, have a straightforward chemical structure that the body processes quickly. This leads to a rapid spike in blood sugar followed by a sharp drop. Complex carbohydrates, found in whole grains, beans, and vegetables, contain fiber and take longer to digest. They raise blood sugar more gradually, producing a slower, steadier curve rather than a sharp peak.
This difference is why two meals with the same number of carbohydrate grams can have very different effects on your blood sugar. A bowl of white rice and a bowl of lentils may contain similar carbohydrate totals, but the lentils will produce a much flatter glucose response because of their fiber content and slower digestion rate.
Protein’s Smaller Role
Protein can also raise blood glucose, though far less than most people expect. Of the 20 amino acids found in protein, all but one can be converted into glucose through a process in the liver. In theory, 100 grams of protein could yield 50 to 80 grams of glucose. In practice, your body converts much less.
When researchers tracked what happened after people ate 50 grams of cottage cheese protein, only about 9.7 grams of glucose actually entered the bloodstream over eight hours. In people with type 2 diabetes, 50 grams of beef protein added only about 2 grams of glucose to the circulation over the same period. The liver has a built-in regulatory system that prevents large protein meals from flooding the blood with glucose, independent of insulin or glucagon. So while protein does contribute some glucose, the effect is modest and slow.
Stress Hormones and the Liver
Stress raises blood sugar even when you haven’t eaten anything. When you’re under physical or emotional stress, your body releases cortisol and other stress hormones that act directly on the liver. Cortisol triggers the liver to produce new glucose from non-carbohydrate sources like amino acids and fat byproducts, and it also stimulates the breakdown of stored glycogen into glucose.
This system evolved to fuel a fight-or-flight response, giving muscles quick access to energy. The problem is that chronic stress keeps this process running long after any physical threat has passed. Prolonged cortisol exposure increases glucose production in the liver and simultaneously makes cells more resistant to insulin. Over time, this combination leads to persistently higher blood sugar and reduced insulin sensitivity. It also explains why people going through extended periods of anxiety, work stress, or difficult life events sometimes see their glucose readings climb without any dietary changes.
Illness and Infection
Getting sick almost always raises blood sugar, sometimes dramatically. During an infection, immune cells ramp up their metabolism and need more glucose to fight off pathogens. At the same time, inflammatory signals released by the immune system reduce insulin sensitivity throughout the body. This is actually a coordinated strategy: the body makes cells somewhat resistant to insulin so that more glucose stays in the bloodstream, available for immune cells that need it.
Pro-inflammatory signals like TNF and interferon-gamma are key players in this process. They increase insulin resistance even in people who normally have no trouble managing blood sugar. This is why a common cold, flu, urinary tract infection, or any inflammatory illness can cause glucose readings to spike for days. For people with diabetes, sick days require closer monitoring because this immune-driven insulin resistance stacks on top of their existing glucose management challenges.
Medications, Especially Steroids
Corticosteroids like prednisone are the most common drug-related cause of high blood sugar. They work through the same pathways as your body’s natural cortisol, ramping up glucose production in the liver while making cells less responsive to insulin. The effect is significant: in hospital settings, 86% of patients receiving high-dose steroids experience at least one episode of elevated blood sugar, and 48% maintain an average glucose above 140 mg/dL.
Even outpatient use carries risk. Among people with no prior history of blood sugar problems, steroid therapy causes new-onset hyperglycemia in roughly 34% to 56% of patients, with glucose levels rising as much as 68% above baseline. This can happen with oral steroids, injected steroids, or even topical and inhaled forms at high doses. Other medications that raise blood sugar include certain blood pressure drugs, some antipsychotics, and specific immunosuppressants.
High-Intensity Exercise
Moderate aerobic exercise, like walking or cycling at a comfortable pace, generally lowers blood sugar by helping muscles absorb glucose without needing much insulin. But intense anaerobic exercise, like sprinting, heavy weightlifting, or competitive sports, can temporarily spike it.
During intense bursts of effort, the body releases catecholamines (adrenaline and noradrenaline) that signal the liver to dump stored glucose into the bloodstream. The liver releases glucose faster than muscles can absorb it, so blood sugar rises. Research shows glucose levels increase significantly right after anaerobic exercise and take about two hours to normalize. This spike is temporary and not harmful for most people, but it can be confusing if you check your blood sugar right after a hard workout and see a higher number than you expected.
Sleep Deprivation
Even a single week of shortened sleep measurably impairs your body’s ability to handle glucose. In a study of healthy men, restricting sleep for one week reduced insulin sensitivity by about 20% on one measure and 11% on another. That means the same amount of insulin moved less sugar out of their blood, leaving glucose levels elevated.
This effect doesn’t require extreme sleep loss. Cutting sleep from eight hours to five or six, the kind of restriction that many people consider normal during a busy week, is enough to shift glucose metabolism. Over time, chronically short sleep contributes to higher fasting glucose and increased diabetes risk. The mechanism involves changes in stress hormones, appetite-regulating hormones, and the nervous system’s balance between its “rest and digest” and “fight or flight” branches.
The Dawn Phenomenon
Many people notice their blood sugar is higher first thing in the morning than it was at bedtime, even though they haven’t eaten for hours. This is the dawn phenomenon, a well-documented pattern driven by your body’s internal clock. In the early morning hours, typically between 4 and 8 a.m., the liver ramps up glucose production through both glycogen breakdown and new glucose creation.
Growth hormone, which peaks during sleep and into the early morning, plays a role by opposing insulin’s effects. Cortisol also rises naturally in the pre-dawn hours as part of the body’s wake-up process. In people without diabetes, the pancreas simply releases more insulin to compensate. In people with type 2 diabetes, the pancreas can’t produce enough extra insulin to counter this surge, so blood sugar drifts upward before breakfast.
Dehydration
When you’re dehydrated, blood sugar readings rise for a straightforward reason: less water in the blood means the same amount of glucose is dissolved in a smaller volume, producing a higher concentration. But dehydration also has active metabolic effects beyond simple concentration. It increases levels of a blood pressure-regulating hormone called vasopressin, which plays a direct role in glucose control. Higher vasopressin signals the liver to produce more glucose.
Dehydration also raises the overall concentration of dissolved particles in the blood, which affects how cells handle insulin and glucagon. Cell shrinkage caused by higher extracellular concentration shifts metabolism in a direction that favors glucose release rather than glucose storage. Staying well hydrated won’t cure blood sugar problems, but chronic low water intake can quietly nudge glucose readings higher over time.
Caffeine
Caffeine reduces your body’s sensitivity to insulin by roughly 13 to 20%, meaning cells don’t respond to insulin as efficiently after you consume it. The mechanism is partially related to adrenaline, since caffeine triggers adrenaline release, but researchers have found that adrenaline alone doesn’t fully explain caffeine’s effect. When study participants were given adrenaline infusions that matched the levels produced by caffeine, the reduction in insulin sensitivity was smaller, suggesting caffeine acts through additional pathways.
For most people, this effect is modest and temporary. Regular coffee drinkers develop some tolerance. But if you’re monitoring your blood sugar closely and notice higher readings in the morning, caffeine from coffee or energy drinks could be a contributing factor worth testing by switching to decaf for a few days and comparing your numbers.