Many things raise blood sugar, and food is only one of them. Your liver, hormones, stress levels, sleep habits, hydration, medications, and even the type of exercise you do all play a role. Understanding these triggers helps explain why blood sugar can spike even when your diet hasn’t changed.
For reference, a normal fasting blood sugar is below 100 mg/dL. The prediabetic range falls between 100 and 125 mg/dL, and a fasting reading of 126 mg/dL or higher on two separate tests indicates diabetes, per the American Diabetes Association’s 2025 standards.
Carbohydrates and How Quickly They Hit
Carbohydrates are the most direct driver of blood sugar. Digestion starts in your mouth, where enzymes begin breaking down starches. As food moves through your digestive system, carbohydrates are split into simple sugars that pass through the intestinal wall into your bloodstream. The speed of that process depends on the type of carbohydrate, how much fiber or fat accompanies it, and how heavily processed the food is.
Simple sugars in candy, juice, or soda enter the bloodstream fast, sometimes producing a noticeable spike within 15 to 30 minutes. Complex carbohydrates in whole grains, beans, and vegetables break down more slowly, producing a gentler, more gradual rise. But “complex” doesn’t always mean “slow.” Maltodextrin, a starch-derived additive found in protein bars, flavored yogurt, salad dressings, and many packaged snacks, is technically a complex carbohydrate. Yet research shows it digests and absorbs at a rate virtually identical to pure glucose, producing the same insulin response. Reading ingredient labels matters, because maltodextrin appears in foods most people wouldn’t consider sugary.
White bread, white rice, instant oatmeal, and breakfast cereals also tend to raise blood sugar quickly despite not tasting sweet. The refining process strips away fiber, which normally slows digestion. Pairing these foods with protein, fat, or fiber (adding nuts to oatmeal, for instance) blunts the spike.
Your Liver Makes Glucose on Its Own
You don’t need to eat anything for your blood sugar to rise. Your liver stores glucose in a compact form called glycogen and releases it between meals and overnight to keep your brain and muscles fueled. Two processes handle this: one breaks down stored glycogen, and the other builds brand-new glucose from non-carbohydrate sources like amino acids and fat byproducts.
Several hormones signal the liver to release glucose. Glucagon, produced by the pancreas, is the primary trigger during fasting and exercise. Adrenaline and noradrenaline from the adrenal glands do the same thing during stress or physical exertion. Growth hormone and cortisol make cells less responsive to insulin while also stimulating the liver to produce more glucose. Even serotonin, best known for its role in mood, directly increases glucose production in liver cells. All of these hormones can push blood sugar up without any food involved.
Stress, Both Physical and Mental
When your body perceives a threat, whether it’s a work deadline, an argument, surgery, or an infection, it shifts into a mode designed to flood your bloodstream with quick energy. Insulin levels drop. Glucagon and adrenaline levels rise, prompting the liver to dump glucose. Simultaneously, cortisol and growth hormone make your muscle and fat tissues resist insulin, so that glucose stays available in the blood rather than being pulled into cells.
This is helpful if you need to run from danger. It’s less helpful when the “threat” is chronic job stress or a lingering illness. Infections, injuries, and even common colds trigger this same hormonal cascade, which is why people with diabetes often see their numbers climb when they’re sick. Emotional stress works through the same pathways: cortisol rises, insulin sensitivity drops, and the liver releases more glucose than the body can clear.
Sleep Deprivation
Even a single night of poor sleep measurably changes how your body handles glucose. In a study of healthy subjects, cutting sleep to roughly 4 hours for just one night reduced the body’s ability to clear glucose from the bloodstream by about 25%. The liver became less responsive to insulin’s signal to stop producing glucose, and muscle and fat tissues pulled less sugar out of the blood. These changes occurred in people with no history of diabetes.
The effect compounds over time. Chronic short sleep, consistently getting fewer than six hours, is associated with higher fasting glucose and greater insulin resistance. If you’ve noticed your blood sugar creeping up without dietary changes, sleep is one of the first things worth examining.
Dehydration
Not drinking enough water raises blood sugar through multiple mechanisms, not just by concentrating the sugar already in your blood. When blood volume drops, your body activates a hormonal system that regulates blood pressure and water balance. One branch of this system releases a hormone called vasopressin, which directly stimulates the liver to break down glycogen and produce new glucose. Vasopressin also triggers cortisol release, which further drives glucose production.
A second branch elevates aldosterone, a hormone that disrupts normal insulin signaling and slows the removal of glucose from the bloodstream. The combined result is that mild, chronic underhydration can meaningfully impair glucose regulation, particularly in people who already have type 2 diabetes. Staying well-hydrated is one of the simplest ways to support stable blood sugar.
High-Intensity Exercise
Moderate aerobic exercise, like walking or cycling at a conversational pace, typically lowers blood sugar by increasing the rate at which muscles absorb glucose. But intense anaerobic exercise, such as sprinting, heavy weightlifting, or competitive sports, can temporarily raise it. The mechanism is straightforward: high-intensity effort triggers a surge of adrenaline and noradrenaline, which signal the liver to release glucose faster than the muscles can use it.
This spike is usually short-lived, and blood sugar tends to come down within an hour or two as the body returns to normal. For people with diabetes, it helps to know that a post-workout reading of 180 or 200 mg/dL after heavy lifting doesn’t necessarily mean something went wrong. The long-term benefits of strength training on insulin sensitivity outweigh the temporary bump.
Medications That Raise Glucose
Corticosteroids are the most well-known offenders. Even a single injection into a joint or tendon can raise fasting blood sugar significantly. In one study of diabetic patients receiving a steroid injection in the hand, blood glucose rose an average of 43 mg/dL the next morning. Another study found an average increase of 73% above baseline the day after a trigger finger injection. Oral steroids like prednisone, often prescribed for asthma flares, autoimmune conditions, or allergic reactions, have an even more pronounced and sustained effect.
Other medication classes that can raise blood sugar include certain blood pressure drugs (particularly thiazide diuretics), some antipsychotic medications, and niacin (vitamin B3) at high doses. If you take any of these and notice rising glucose levels, the medication may be a contributing factor worth discussing with your prescriber.
The Morning Spike
Many people notice their blood sugar is higher when they wake up than when they went to bed, even if they didn’t eat overnight. Two distinct phenomena explain this.
The dawn phenomenon affects most people to some degree. In the early morning hours, roughly between 4 a.m. and 8 a.m., the body releases a wave of hormones (cortisol, growth hormone, glucagon) that increase insulin resistance and stimulate the liver to produce glucose. This is a normal part of the body’s wake-up process, designed to give you energy for the day. In people whose insulin production or medication coverage can’t keep up, blood sugar rises noticeably.
The Somogyi effect is different. It occurs when too much insulin (from medication, not the body’s own production) causes blood sugar to drop too low during the night, triggering a rebound surge of counter-regulatory hormones that overshoot and push glucose high by morning. Measuring blood sugar between 3 a.m. and 5 a.m. for several nights, or using a continuous glucose monitor, helps distinguish between the two. The treatments are opposite: the dawn phenomenon may call for adjusting the timing of evening medication, while the Somogyi effect requires reducing it.