Your blood sugar rises whenever glucose enters your bloodstream faster than your body can move it into cells. Food is the most obvious trigger, but stress, poor sleep, illness, certain medications, and even dehydration can push your numbers higher. Understanding each of these causes helps you spot patterns and make sense of readings that seem to come out of nowhere.
How Your Body Manages Blood Sugar
Your pancreas produces two key hormones that work like a thermostat for blood sugar. Insulin lowers it by shuttling glucose out of the blood and into your muscles, fat, and liver cells, where it’s either burned for energy or stored. Glucagon does the opposite: when blood sugar drops too low, it signals the liver to convert stored glucose into a usable form and release it back into the bloodstream. Glucagon also triggers the liver to create brand-new glucose from protein and other non-carbohydrate sources.
When this system works smoothly, the two hormones counterbalance each other and blood sugar stays in a tight range. Fasting levels for most adults with diabetes should land between 80 and 130 mg/dL, and readings one to two hours after eating should stay below 180 mg/dL, according to the American Diabetes Association. For people without diabetes, those ranges are even narrower. Problems start when something disrupts either side of this balance: too much glucose coming in, not enough insulin going out, or cells that stop responding to insulin properly.
Carbohydrates and the Foods You Eat
Carbohydrates have the most direct effect on blood sugar because your body breaks them down into glucose. 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 scoring 100. In general, the more processed a food is, the higher its glycemic index. The more fiber or fat a food contains, the lower it scores.
White bread, sugary cereals, and fruit juice cause rapid spikes because they’re digested quickly and deliver a flood of glucose all at once. Whole grains, legumes, and most vegetables release glucose gradually, giving insulin time to keep up. That said, Harvard Health notes that the total amount of carbohydrate in a food is actually a stronger predictor of what will happen to your blood sugar than its glycemic index alone. A small portion of white rice may raise your blood sugar less than a large bowl of oatmeal, simply because there’s less total carbohydrate on the plate. Paying attention to both the type and the quantity of carbs you eat gives you the most complete picture.
Stress Hormones and the Fight-or-Flight Response
When you’re stressed, whether it’s a work deadline, a family argument, or financial pressure, your body treats it like a physical threat. It floods your bloodstream with cortisol and adrenaline, both of which raise blood sugar through the same basic mechanism: they tell your liver to dump stored glucose into the bloodstream. Cortisol goes a step further by prompting the liver to manufacture new glucose from proteins, a process that can keep blood sugar elevated for hours even if you haven’t eaten anything.
This made sense for our ancestors, who needed a burst of energy to outrun a predator. It’s less helpful when the “threat” is a traffic jam. For people with diabetes or insulin resistance, these stress-driven spikes can be especially hard to manage because the extra glucose has nowhere to go efficiently. Chronic, ongoing stress keeps cortisol elevated for long stretches, which means your liver may be quietly adding glucose to your blood around the clock.
Why Blood Sugar Rises Overnight
Waking up with higher blood sugar than you had at bedtime is surprisingly common, and there are two main explanations. The more common one is called the dawn phenomenon. In the early morning hours, your body naturally releases cortisol and growth hormone to prepare you for waking up. These hormones raise blood sugar as part of your normal circadian rhythm, even if you haven’t eaten since dinner.
The less common explanation is called the Somogyi effect. This happens when blood sugar drops too low during the night, usually because of injected insulin. Your body responds to the low by releasing a rescue squad of hormones, including adrenaline, glucagon, cortisol, and growth hormone, all of which cause the liver to release a large amount of stored glucose. The result is a rebound high by morning. The key difference: the dawn phenomenon starts from normal blood sugar, while the Somogyi effect starts from a low. A continuous glucose monitor can reveal which pattern is happening by tracking your levels through the night.
Sleep Deprivation and Insulin Resistance
Even a few nights of poor sleep can make your cells less responsive to insulin, meaning glucose builds up in your bloodstream instead of being absorbed. The research on this is remarkably consistent. Multiple clinical trials have measured the effect, and the numbers are striking: insulin sensitivity drops by roughly 16 to 29 percent after just a few nights of restricted sleep. One study found that a single night of sleep deprivation reduced insulin sensitivity by 21 percent, with no compensating increase in insulin production to make up the difference.
The good news is that this effect appears to be reversible. Research has shown that people who extended their sleep beyond six hours per night saw meaningful improvements in fasting insulin resistance, insulin secretion, and the function of the cells that produce insulin. Sleep isn’t just rest for your brain; it’s active maintenance time for your metabolism.
Illness and Infection
Getting sick, whether it’s a cold, the flu, a urinary tract infection, or something more serious, almost always raises blood sugar. Your immune system releases hormones as part of its defense response, and those hormones increase blood sugar temporarily to fuel the fight against the invader. At the same time, inflammation from the illness makes your cells more resistant to insulin, creating a double hit. This is why people with diabetes are often told to monitor their blood sugar more frequently when they’re under the weather. Even a mild illness can push numbers well above their usual range for days.
Intense Exercise
Moderate aerobic exercise, like walking or cycling, generally lowers blood sugar by helping your muscles absorb glucose. But intense, anaerobic exercise can do the opposite. Heavy weightlifting, sprints, and competitive sports trigger the release of adrenaline, which stimulates your liver to release glucose, just as it does during stress. The spike is usually temporary and blood sugar tends to drop in the hours afterward, but it can be confusing if you check your levels right after a hard workout and see a number higher than when you started.
Medications That Raise Blood Sugar
Several common medications can push blood sugar higher, even in people who don’t have diabetes. The most well-known offenders are corticosteroids (like prednisone), which mimic cortisol’s effect on the liver and can cause dramatic spikes. Thiazide diuretics, often prescribed for high blood pressure, can also raise glucose levels. Birth control pills and progesterone-based contraceptives have a modest effect in some people. Certain antipsychotic medications and high-dose niacin (a B vitamin sometimes used for cholesterol) round out the list, though the blood sugar effect of niacin tends to diminish after a few months of use.
If you notice your blood sugar climbing after starting a new medication, it’s worth checking whether it’s on this list. The effect varies widely from person to person, and sometimes a dosage adjustment or alternative medication can solve the problem.
Dehydration
When you’re dehydrated, you have less water in your bloodstream, which means the glucose that’s there becomes more concentrated. Your actual glucose production may not have changed at all, but your blood sugar reading goes up simply because the same amount of sugar is dissolved in less fluid. This is especially relevant in hot weather, during illness (when you may not feel like drinking), or after exercise. Staying well-hydrated won’t cure high blood sugar, but it can prevent readings from being artificially inflated and help your kidneys clear excess glucose more efficiently.