Blood glucose rises when your body either can’t make enough insulin, can’t use insulin effectively, or gets flooded with sugar-releasing signals from hormones, food, or medications. A normal fasting level sits below 100 mg/dL. Between 100 and 125 mg/dL is considered prediabetes, and 126 mg/dL or higher on two separate tests indicates diabetes.
How Insulin Normally Controls Blood Sugar
After you eat, your pancreas releases insulin, which acts like a key that unlocks your cells so glucose can enter and be used for energy. The process depends on a specific transporter protein called GLUT4 that moves to the surface of muscle and fat cells when insulin signals them. When this system works well, glucose clears from your bloodstream within a couple of hours after a meal, and your levels stay in a tight range.
In insulin resistance, that signaling chain breaks down. Your cells stop responding efficiently to insulin’s signal, so GLUT4 transporters don’t reach the cell surface in adequate numbers. Glucose stays stranded in your bloodstream. Your pancreas compensates by pumping out more insulin, which works for a while, but over months or years the pancreas can’t keep up. That’s the progression from insulin resistance to prediabetes to type 2 diabetes.
Food Choices and Meal Timing
Not all carbohydrates hit your bloodstream at the same speed. Refined carbs like white bread, sugary drinks, and white rice break down quickly and cause a sharp glucose spike. Whole grains, legumes, and most vegetables release glucose more gradually because their fiber slows digestion. The total amount of carbohydrate in a meal matters just as much as the type. A small portion of white rice raises glucose less than a massive bowl of brown rice, even though brown rice is the “healthier” option per serving.
Pairing carbohydrates with protein, fat, or fiber slows the rate glucose enters your blood. Eating a piece of fruit with a handful of nuts, for example, produces a flatter glucose curve than eating the fruit alone. Meal sequence matters too. Starting with vegetables or protein before eating starchy foods can blunt the post-meal spike.
Stress Hormones and Your Liver
Your body stores glucose in the liver as a reserve fuel called glycogen. When you’re stressed, physically or emotionally, your brain triggers a cascade of hormones that force the liver to dump that stored glucose into your bloodstream. Epinephrine (adrenaline) is the most potent trigger, driving the liver to break down glycogen rapidly. It also stimulates the liver to build new glucose from scratch using amino acids and other raw materials, a process called gluconeogenesis.
Cortisol plays a supporting role by increasing the supply of raw materials the liver needs to manufacture glucose and by building up glycogen stores so there’s more fuel available for the next stress response. In one well-studied model, about 70% of the stress-driven rise in blood sugar came from new glucose production, and 30% from glycogen breakdown. This is why chronic stress, work pressure, anxiety, financial worry, or ongoing pain can keep your glucose stubbornly elevated even when your diet looks clean.
The Early Morning Glucose Rise
Many people with diabetes notice their blood sugar is higher when they wake up than when they went to bed. This is usually the dawn phenomenon. Between roughly 3 a.m. and 5 a.m., your body’s natural insulin secretion drops to its lowest point. At the same time, growth hormone, cortisol, and adrenaline rise as part of your normal wake-up process. These hormones push glucose out of the liver. In someone without diabetes, the pancreas simply releases more insulin to match. In someone with diabetes or prediabetes, that compensating insulin surge doesn’t happen, and blood sugar climbs between about 5 a.m. and 8 a.m.
A less common cause is a rebound from overnight low blood sugar. If glucose drops too low during the night (often from too much evening insulin), the body overcorrects with a flood of counter-regulatory hormones, producing a high reading by morning. The distinction matters because the solutions are opposite: the dawn phenomenon often calls for adjusted medication timing, while a rebound low calls for less medication or a bedtime snack.
Sleep and Insulin Sensitivity
Even a single night of poor sleep measurably changes how your body handles glucose. In a controlled study, one night of partial sleep restriction (sleeping roughly four hours instead of eight) reduced insulin sensitivity by 14 to 21%. That means the body needed significantly more insulin to clear the same amount of glucose from the bloodstream. The participants didn’t eat differently or exercise less. Sleep loss alone was enough to shift their metabolism toward higher blood sugar.
This effect compounds over time. Chronic short sleep, regularly getting less than six hours, is consistently linked to higher fasting glucose and a greater risk of developing type 2 diabetes. Night-shift work, sleep apnea, and even inconsistent sleep schedules (sleeping in on weekends, waking early on weekdays) can all degrade glucose control.
Dehydration’s Surprising Role
When you’re dehydrated, your blood volume drops and its concentration increases, which can make glucose readings appear higher simply because there’s less water diluting the sugar. But the effect goes beyond simple concentration. Dehydration triggers the release of a hormone called vasopressin, which tells the kidneys to conserve water. Vasopressin also acts directly on the liver, stimulating it to produce and release more glucose. On top of that, vasopressin activates the stress hormone axis, increasing cortisol and adrenaline, both of which further push glucose up.
A small study of people with type 2 diabetes found that just three days of restricted water intake led to impaired glucose response, higher cortisol during glucose testing, and reduced insulin sensitivity compared to when the same people were properly hydrated. High blood sugar itself worsens dehydration because excess glucose spills into urine and pulls water with it, creating a vicious cycle.
Medications That Raise Blood Sugar
Several commonly prescribed medications can push glucose higher as a side effect. Corticosteroids (like prednisone), often used for asthma, autoimmune conditions, and joint inflammation, are among the most well-known offenders. They increase liver glucose production and make cells more resistant to insulin.
Thiazide diuretics, frequently prescribed for high blood pressure, raise glucose through a less obvious path. They lower potassium levels, and potassium is needed for the pancreas to secrete insulin properly. Low potassium impairs insulin release and can independently worsen insulin resistance. The effect is dose-dependent, so lower doses cause less glucose disruption. Other medications that can elevate blood sugar include certain antipsychotics, some beta-blockers, and niacin at high doses. If you notice glucose creeping up after starting a new medication, that connection is worth exploring.
Exercise: It Can Go Both Ways
Moderate aerobic exercise, like walking or cycling, typically lowers blood sugar by helping muscles pull glucose from the bloodstream without needing as much insulin. But intense exercise can temporarily raise it. Heavy weightlifting and high-intensity interval training trigger a burst of adrenaline and cortisol that signals the liver to release glucose for quick energy. Your muscles are using fuel rapidly, but the liver dumps glucose even faster, so your reading spikes.
This spike is usually short-lived, lasting 30 minutes to a couple of hours after the workout, and glucose often settles to a lower level than where it started. For people with diabetes who monitor frequently, it helps to know this pattern is normal and not a reason to avoid intense exercise. Over weeks and months, both aerobic and resistance training improve insulin sensitivity and lower average glucose levels.
Other Common Triggers
Illness and infection raise glucose because your immune system’s inflammatory response activates the same stress hormones that prepare the body for fight-or-flight. Even a mild cold can push readings higher for several days. Surgery, dental procedures, and significant pain have similar effects.
Smoking increases insulin resistance, and the effect is dose-dependent. The more cigarettes per day, the greater the impairment. Alcohol is unpredictable: sugary cocktails and beer can spike glucose, while moderate alcohol on an empty stomach can actually cause a dangerous drop by blocking the liver’s glucose production. The combination of alcohol and diabetes medications requires particular caution.
Hormonal shifts during the menstrual cycle, pregnancy, menopause, and thyroid disorders all influence glucose regulation. Progesterone, which rises in the second half of the menstrual cycle, temporarily increases insulin resistance, which is why some women notice higher readings in the week or two before their period.