High blood sugar happens when glucose builds up in your bloodstream faster than your body can move it into cells for energy. A normal fasting blood sugar is below 100 mg/dL, and anything from 100 to 125 mg/dL falls into the prediabetes range. If your fasting level hits 126 mg/dL or higher on two separate tests, that’s diabetes. But plenty of things besides diabetes can push your numbers up, and some of them are surprisingly mundane.
How Blood Sugar Gets Stuck in the Bloodstream
Your body runs on a simple loop: you eat, your blood sugar rises, your pancreas releases insulin, and insulin acts like a key that unlocks your cells so glucose can enter. When that system breaks down, glucose stays in the blood instead of fueling your muscles, liver, and brain.
The breakdown usually happens in one of two places. First, your cells can become resistant to insulin. When fat accumulates inside muscle and liver cells where it doesn’t belong, it interferes with insulin’s ability to signal those cells to open up and absorb glucose. Your pancreas compensates by pumping out more insulin, but over time it can’t keep up. Second, your liver can overproduce glucose on its own. Excess fatty acids flowing to the liver activate a pathway that ramps up the liver’s internal glucose-manufacturing process, flooding your bloodstream even when you haven’t eaten. In most people with consistently high blood sugar, both problems are happening at once.
Stress Hormones and Cortisol
Stress is one of the most underestimated drivers of high blood sugar. When you’re under physical or emotional stress, your body releases cortisol, a hormone that tells the liver to push more glucose into the bloodstream so you’ll have energy to respond to a perceived threat. That made sense when threats involved running from predators. It’s less helpful when the stress is a deadline at work or a bad night of sleep.
Cortisol also makes your muscles more resistant to insulin, partly by blocking the transport molecules that carry glucose into muscle cells. On top of that, it breaks down muscle and fat tissue, releasing raw materials the liver uses to manufacture even more glucose. Chronic stress keeps cortisol elevated for hours or days at a time, which can create a cycle where your blood sugar stays higher than it should even if your diet hasn’t changed.
Poor Sleep Has a Measurable Effect
A single night of bad sleep can reduce your insulin sensitivity by roughly 20 to 25%. That means the same meal you ate yesterday will produce a higher blood sugar spike today if you slept poorly last night. Multiple studies using different measurement techniques have confirmed this range, with one finding a 29% drop in the ability of muscles specifically to respond to insulin after restricted sleep. Your pancreas doesn’t automatically produce more insulin to compensate, so the glucose just lingers in your blood longer than usual.
This helps explain why people who work night shifts, have sleep apnea, or regularly get fewer than six hours of sleep face significantly higher rates of prediabetes and type 2 diabetes over time.
Medications That Raise Blood Sugar
Several common drug classes can push blood sugar up as a side effect. The biggest offenders are corticosteroids (like prednisone), which mimic the effect of cortisol and create widespread insulin resistance while also impairing the insulin-producing cells in your pancreas. Even a short course of steroids for an allergic reaction or joint pain can spike your numbers noticeably.
Other medications linked to higher blood sugar include thiazide diuretics (often prescribed for blood pressure), beta-blockers, statins, certain antipsychotic medications, and some antibiotics in the fluoroquinolone family. The mechanisms vary. Some reduce insulin secretion, some increase insulin resistance, and some are directly toxic to pancreatic cells at high doses. If your blood sugar crept up after starting a new medication, that’s a connection worth investigating with your prescriber.
Illness and Infection
Getting sick, even with a common cold or urinary tract infection, can raise blood sugar. Your immune system releases inflammatory signaling molecules that make cells throughout your body more resistant to insulin. This is actually a deliberate survival mechanism: your body restricts glucose uptake by regular cells so that immune cells fighting the infection have more fuel available.
During mild infections, your pancreas often compensates by producing extra insulin, so you may not notice a change. But during more severe illnesses, the compensation falls short and blood sugar rises. This is why people with diabetes are often told to monitor their levels more carefully when they’re sick. The effect is temporary and typically resolves as the infection clears.
Dehydration Concentrates Your Blood Sugar
This one catches people off guard. When you’re dehydrated, you have less water in your bloodstream, which means the same amount of glucose is dissolved in a smaller volume of fluid. Your blood sugar reading goes up even though you haven’t consumed any extra sugar. The CDC lists dehydration as one of the surprising things that spike blood sugar. Drinking water won’t cure an underlying problem, but staying well hydrated can prevent artificially inflated readings and helps your kidneys clear excess glucose more efficiently.
Why Blood Sugar Spikes in the Morning
Waking up with high blood sugar when you haven’t eaten for eight hours feels counterintuitive, but it’s common. The most frequent explanation is the dawn phenomenon: between roughly 4 a.m. and 8 a.m., your body releases a surge of cortisol, growth hormone, and other hormones that tell the liver to release stored glucose so you’ll have energy to start the day. In people whose insulin response is already compromised, that surge pushes fasting numbers higher than expected.
A less common cause, relevant mainly for people on insulin, is called the Somogyi effect. In this case, blood sugar drops too low overnight (often because of an evening insulin dose), and the body overcompensates by flooding the bloodstream with glucose-raising hormones like adrenaline and glucagon. The result looks identical in the morning: high blood sugar. The difference matters because the fix is opposite. The dawn phenomenon may need more medication coverage in the early hours, while the Somogyi effect may need less insulin the night before. Checking your blood sugar at 2 or 3 a.m. on a few occasions, or using a continuous glucose monitor, can tell the two apart.
Exercise Can Temporarily Raise It Too
Moderate exercise like walking or cycling generally lowers blood sugar by helping muscles absorb glucose. But intense exercise, think sprinting, heavy weightlifting, or competitive sports, can temporarily raise it. During high-intensity effort, your body releases a surge of adrenaline that directly stimulates the liver to dump stored glucose into the bloodstream. This happens because the normal hormonal signals (insulin and glucagon adjustments) aren’t enough to fuel muscles during all-out exertion, so adrenaline steps in to bridge the gap.
The spike is usually short-lived. Blood sugar typically comes back down within an hour or two after intense exercise ends, and regular physical activity improves insulin sensitivity over time. If you’re seeing high readings immediately after a hard workout, that’s the adrenaline response at work, not a sign that exercise is hurting you.
Food Timing and Composition
Beyond the obvious (eating a lot of sugar or refined carbohydrates), the way you structure meals affects how high your blood sugar climbs afterward. A normal post-meal reading should stay below 140 mg/dL two hours after eating. Readings between 140 and 199 mg/dL at that mark suggest prediabetes, and anything over 200 mg/dL indicates diabetes.
Eating carbohydrates by themselves, without protein, fat, or fiber, produces the sharpest blood sugar spikes because there’s nothing to slow digestion. Large meals produce bigger spikes than smaller ones. Eating late at night tends to produce higher glucose responses than the same meal eaten earlier in the day, partly because insulin sensitivity naturally decreases in the evening. Skipping meals can also backfire: prolonged fasting triggers the liver to produce glucose on its own, and when you finally eat, your body may be less prepared to handle the incoming load efficiently.