Blood sugar rises when glucose enters your bloodstream faster than your body can move it into cells, or when your liver releases stored glucose you don’t need. For people without diabetes, fasting blood sugar typically stays between 80 and 130 mg/dL, and levels after eating should peak below 180 mg/dL. When blood sugar climbs above those thresholds, a wide range of causes could be responsible, from what you ate to how you slept last night.
How Your Body Normally Controls Blood Sugar
Your blood sugar is managed by two hormones working in opposition. After you eat, your pancreas releases insulin, which signals cells throughout your body to absorb glucose from the bloodstream for energy. Between meals, your pancreas releases a different hormone that does the opposite: it tells your liver to convert stored glycogen back into glucose and release it, keeping your blood sugar from dropping too low. It also triggers your liver to manufacture brand-new glucose from non-sugar building blocks.
High blood sugar happens when this system falls out of balance. Either your cells stop responding well to insulin (insulin resistance), your pancreas can’t produce enough insulin, or something is pushing your liver to dump extra glucose into your blood. Most causes of high blood sugar trace back to one of these three disruptions.
Food and Carbohydrates
Carbohydrates are the primary dietary driver of blood sugar. Your digestive system breaks carbs down into glucose, which enters your bloodstream. But not all carbs hit the same way. Foods with a high glycemic index, like white bread, sugary drinks, and white rice, raise blood sugar higher, faster, and for a longer time than foods with a lower glycemic index, such as whole grains, legumes, and most vegetables.
The difference comes down to how quickly your gut can break the food apart. Simple, refined carbohydrates are already close to pure glucose, so they flood the bloodstream in minutes. Complex carbohydrates wrapped in fiber take longer to digest, producing a slower, more gradual rise. Eating carbs alongside fat, protein, or fiber also slows digestion and blunts the spike. This is why a bowl of white rice alone can push blood sugar much higher than the same rice served with vegetables and chicken.
Stress Hormones
Stress, whether physical or emotional, triggers the release of adrenaline and cortisol. Both hormones raise blood sugar through the same basic mechanism: they tell your liver to release stored glucose and manufacture new glucose, while simultaneously making your cells less responsive to insulin. This is an ancient survival response designed to flood your muscles with quick energy during a threat.
The problem is that modern stress rarely requires a burst of physical energy. Work deadlines, financial worry, and sleep loss all activate the same hormonal cascade, pushing blood sugar up with no physical activity to burn through it. Chronic stress keeps cortisol elevated for extended periods, which can maintain mild insulin resistance even in otherwise healthy people.
Intense Exercise
This one surprises a lot of people. While most aerobic exercise lowers blood sugar, high-intensity activities like sprinting, heavy weightlifting, and competitive sports can temporarily raise it. The cause is adrenaline. Your body interprets the intense effort as a physical stressor, triggers adrenaline release, and your liver responds by dumping glucose into the bloodstream to fuel the effort.
This spike is usually short-lived, and blood sugar typically drops below its starting point within an hour or two as your muscles replenish their energy stores. If you track your blood sugar and notice post-workout spikes, relaxation techniques like paced breathing before and during exercise can reduce the adrenaline effect.
Poor Sleep
Sleep deprivation directly impairs how your cells respond to insulin. In a controlled study, healthy men who slept only 5 hours per night for one week experienced a 20% reduction in insulin sensitivity compared to when they slept a full night. That means their bodies needed significantly more insulin to move the same amount of glucose out of the bloodstream.
Even a few nights of short sleep can start this process. And the effect compounds: poor sleep also increases cortisol levels the following day, adding a stress-hormone contribution on top of the reduced insulin sensitivity. If your blood sugar has been running higher than expected, your sleep schedule is one of the first things worth examining.
Illness and Infection
When you’re sick, your immune system releases inflammatory signaling molecules called cytokines to fight off infection. These same molecules interfere with insulin signaling in your cells. One type reduces the number of glucose transporters your cells put on their surface, making it harder for glucose to get inside. Another increases the breakdown of fat stores, which releases compounds that further block insulin’s action.
This is why blood sugar can spike noticeably during a cold, flu, urinary tract infection, or any inflammatory illness. The effect is temporary but can be significant, especially if you already have diabetes or prediabetes. Even a minor infection can push blood sugar 50 to 100 mg/dL higher than your usual range for several days.
Dehydration
When you’re dehydrated, you have less water in your bloodstream, which means the glucose that’s there becomes more concentrated. Your actual amount of glucose may not have changed, but the reading on a meter goes up because it’s measuring concentration, not total quantity. Mild dehydration (losing less than 2% of body weight in water) is enough to produce this effect and also triggers hormonal changes associated with higher blood sugar responses.
Staying well-hydrated is one of the simplest ways to keep blood sugar readings from creeping up unnecessarily, particularly in hot weather or after exercise.
Medications
Several common medication classes raise blood sugar as a side effect. The most well-known are glucocorticoids (steroids like prednisone and dexamethasone), which are prescribed for conditions ranging from asthma to autoimmune disease. All routes of administration can cause hyperglycemia at high doses, but oral steroids carry the highest risk.
Other medications that can push blood sugar up include certain blood pressure drugs in the thiazide diuretic class, some antipsychotic medications, beta blockers, and statins. One large trial found that participants taking the thiazide diuretic chlorthalidone had a 48% higher incidence of developing diabetes compared to those taking a different blood pressure medication. If you’ve started a new medication and noticed higher blood sugar readings, the drug itself could be the cause.
High Morning Blood Sugar
Waking up with elevated blood sugar is common and has two distinct explanations. The first, called the dawn phenomenon, happens because your body naturally secretes the least insulin between midnight and 6 AM. At the same time, hormones that oppose insulin (cortisol and growth hormone) rise in the early morning hours to prepare your body for waking. This combination means your liver releases glucose without enough insulin to keep it in check.
The second explanation applies mainly to people taking insulin. If an evening dose of insulin drops blood sugar too low during the night, the body responds with a surge of counter-regulatory hormones: adrenaline, cortisol, growth hormone, and glucagon. These hormones push the liver to produce glucose aggressively, and blood sugar rebounds to abnormally high levels by morning. The key difference is that this rebound pattern involves a nighttime low followed by a morning high, while the dawn phenomenon involves a gradual rise without any preceding low.
Insulin Resistance and Weight
Excess body fat, particularly around the abdomen, is one of the most significant long-term drivers of high blood sugar. Fat tissue, especially when inflamed, attracts immune cells called macrophages that produce the same inflammatory cytokines released during illness. These molecules chronically impair insulin signaling, forcing the pancreas to produce more and more insulin to keep blood sugar in range.
For a while, the pancreas compensates by ramping up production. Blood sugar stays normal, but insulin levels are elevated. Eventually, the pancreas can’t keep up with the demand, and blood sugar begins to rise. This progression from insulin resistance to prediabetes to type 2 diabetes can take years, and blood sugar may only test as “high” once the process is already well underway. This is why insulin resistance is sometimes called a silent condition: the damage accumulates before the numbers on a blood test start to show it.