Many things raise blood sugar beyond the obvious culprit of sugary foods. Stress, poor sleep, certain medications, dehydration, and even your body’s own hormonal cycles can push glucose levels higher. Understanding these triggers helps you spot patterns and make sense of unexpected readings.
Carbohydrates and How Fast They Hit
Carbohydrates are the primary dietary driver of blood sugar. When you eat them, your body breaks them down into glucose, which enters your bloodstream. But not all carbs behave the same way. The glycemic index (GI) scores foods from 0 to 100 based on how quickly they spike blood sugar, with pure glucose set at 100. White bread, white rice, and sugary cereals score high, meaning they deliver glucose fast. Lentils, most vegetables, and steel-cut oats score low, releasing glucose gradually.
A general rule: the more processed a food is, the higher its GI. The more fiber or fat it contains, the lower. But the GI alone doesn’t tell the whole story. A measure called glycemic load factors in how much carbohydrate a typical serving actually contains. Watermelon, for example, has a high GI but a low glycemic load because a normal portion doesn’t contain much total sugar. Portion size matters as much as the type of carb.
Stress Hormones and the “Fight or Flight” Response
When you’re stressed, whether from a work deadline, an argument, or a physical injury, your body assumes it needs quick energy. Insulin levels drop while adrenaline and glucagon levels rise. This signals your liver to dump stored glucose into your bloodstream. For someone without diabetes, the body compensates and brings levels back to normal fairly quickly. For someone with diabetes or insulin resistance, that extra glucose can linger and cause a noticeable spike.
This isn’t limited to emotional stress. Surgery, infections, and chronic pain trigger the same hormonal cascade. The effect is real and measurable, which is why blood sugar can climb on difficult days even when you haven’t changed what you eat.
The Dawn Phenomenon
If your fasting blood sugar is higher in the morning than it was at bedtime, you’re likely experiencing the dawn phenomenon. Between roughly 4 a.m. and 8 a.m., the body releases a surge of hormones, including growth hormone, cortisol, glucagon, and adrenaline, that naturally increase insulin resistance. The purpose is to give you energy to start the day, but in people with diabetes, this hormonal wave can push morning glucose readings well above target. It’s one of the most common causes of unexplained high fasting numbers.
Sleep Deprivation
Losing sleep does more than make you tired. A study published by the American Diabetes Association found that just one week of sleeping five hours per night reduced insulin sensitivity by 20% in healthy men. That means their cells became significantly worse at pulling glucose out of the blood, even though their bodies were still producing the same amount of insulin. A separate measure in the same study confirmed an 11% reduction using a more precise testing method.
This matters because many people routinely sleep five or six hours and don’t realize it’s affecting their metabolism. The effect compounds over time, contributing to the kind of chronic insulin resistance that raises disease risk.
Medications That Raise Glucose
Several common prescription drugs push blood sugar up as a side effect. Corticosteroids are the biggest offenders. Prednisone, dexamethasone, and hydrocortisone, often prescribed for inflammation, asthma, or autoimmune conditions, typically raise blood sugar within four to eight hours of taking a pill. Larger doses produce larger spikes. Importantly, steroids cause high blood sugar even in people who don’t have diabetes, so if you’re on a short course and notice unusual readings, this is likely why.
Other medications associated with glucose increases include certain diuretics (water pills used for blood pressure), some beta-blockers, and antipsychotic medications. If you’ve started a new prescription and noticed your numbers climbing, the medication may be a contributing factor worth discussing with your prescriber.
Illness and Infection
Being sick almost always raises blood sugar. When your immune system fights an infection, the body releases inflammatory signaling molecules that interfere with insulin’s ability to work. One of these, TNF-alpha, directly impairs insulin signaling in fat tissue and muscle, reducing how much glucose those cells absorb. Another, IL-6, has been shown to induce both elevated blood sugar and a compensatory surge in insulin, both hallmarks of insulin resistance.
This creates a feedback loop: high blood sugar itself increases inflammatory molecule production through oxidative stress, which further worsens insulin resistance. It’s one reason blood sugar can be especially difficult to manage during a cold, flu, or any other illness.
Dehydration
When you’re dehydrated, the total volume of water in your bloodstream drops. This concentrates everything dissolved in it, including glucose. Your body doesn’t necessarily have more sugar circulating. The ratio of sugar to water simply shifts, and a blood test or glucose monitor reads the concentration as higher. Drinking enough water won’t cure high blood sugar from other causes, but dehydration can make an existing elevation look worse than it is and can genuinely affect readings.
Caffeine
For most healthy adults, caffeine doesn’t noticeably change blood sugar. But if you have diabetes, it can alter the way your body uses insulin, potentially raising glucose levels. The threshold seems to be around 200 milligrams, roughly the amount in two standard cups of coffee. Some people with diabetes see a clear effect at that dose, while others see none at all. If your morning numbers seem inconsistent, it’s worth checking whether your coffee intake varies day to day.
Artificial Sweeteners
Zero-calorie sweeteners were long assumed to have no effect on blood sugar since they contain no carbohydrates. A 2022 study published in Cell challenged that assumption. Researchers found that saccharin and sucralose significantly impaired glycemic responses in human participants, meaning their blood sugar after meals rose more than expected. The mechanism appeared to involve changes in gut bacteria: when researchers transplanted the altered microbiomes from human participants into germ-free mice, the mice developed the same impaired glucose responses.
The effect was personalized. Some people responded strongly, others barely at all, and the differences tracked with the specific microbial changes each person developed. Stevia and aspartame showed weaker or no consistent effects across the group. This doesn’t mean you need to avoid all sweeteners, but it does mean they aren’t metabolically invisible for everyone.
Knowing Your Target Numbers
The American Diabetes Association recommends that people with diabetes aim for blood sugar between 80 and 130 mg/dL before meals, and under 180 mg/dL two hours after eating. For adults 60 and older or those with other health conditions, slightly higher pre-meal targets of 100 to 140 mg/dL are sometimes appropriate. If your blood sugar reaches 240 mg/dL or higher, testing for ketones with an over-the-counter urine kit is a smart precaution, since levels that high can signal a more serious metabolic problem.
Tracking your numbers alongside potential triggers, what you ate, how you slept, whether you’re stressed or fighting off a cold, is the most practical way to identify which factors affect you most. Blood sugar is rarely driven by a single cause, and the triggers that matter vary from person to person.