Many things increase blood sugar, and most of them aren’t just the obvious sugary foods. Stress, poor sleep, certain medications, illness, and even dehydration can all push your glucose levels higher. Understanding these triggers helps whether you’re managing diabetes, watching your numbers in the prediabetes range (fasting glucose of 100 to 125 mg/dL), or simply trying to keep your energy stable throughout the day.
Carbohydrates and How Fast They Hit
Carbohydrates are the most direct 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. Foods that are rapidly digested and absorbed, like white bread, white rice, and sugary drinks, cause a fast, steep spike. Foods that break down slowly, like lentils, oats, and most vegetables, produce a smaller, more gradual rise.
This difference comes down to the type of starch, the amount of fiber, and what else is in the food. Soluble fiber (found in oats, beans, and barley) slows the release of glucose into your blood, while fat and protein in a meal also put the brakes on digestion. The total amount of carbohydrate matters just as much as the type. Watermelon, for example, has a high glycemic index of 74, but a 100-gram serving contains so little carbohydrate that its actual blood sugar impact is minimal.
Refined baked goods deserve special attention. Products rich in both fat and refined carbohydrates, like pastries and doughnuts, trigger an insulin response that’s disproportionately high compared to the glucose they produce. In practical terms: the combination of white flour, sugar, and butter hits your metabolic system harder than you’d expect from the carb count alone. Reducing total carbohydrate intake at each meal, and including at least 2 grams of soluble fiber at breakfast, are two of the most effective dietary strategies for keeping post-meal glucose in check.
Stress Hormones and the Liver
Your body doesn’t need food to raise blood sugar. Stress, whether physical or emotional, triggers the release of cortisol and adrenaline. Cortisol signals your liver to manufacture new glucose from stored fat and protein, then dump it into your bloodstream. It also amplifies the blood sugar-raising effects of other hormones like glucagon. Adrenaline does something similar, mobilizing quick energy for what your body perceives as an emergency.
This means a difficult week at work, a family crisis, or chronic anxiety can raise your glucose levels even if your diet hasn’t changed. For people with diabetes, stress-related spikes can be especially confusing because they seem to come from nowhere. The effect is real and measurable: cortisol directly reduces your body’s ability to use insulin effectively, creating a double hit of more glucose entering your blood and less being cleared out of it.
Sleep Deprivation
Even a single night of poor sleep can make your body less responsive to insulin. In a study of healthy subjects, restricting sleep to roughly four hours for just one night reduced the body’s ability to clear glucose from the blood by about 25%. The liver became resistant to insulin’s signal to stop producing glucose, and muscle tissue absorbed less glucose than normal. Both pathways were impaired at the same time.
This wasn’t observed in people with diabetes. It happened in healthy volunteers with no metabolic issues, which suggests that chronic short sleep is a meaningful risk factor for developing blood sugar problems over time. If you regularly sleep fewer than six hours, your fasting glucose may be higher than it would otherwise be, regardless of what you eat.
Illness and Infection
When your body fights an infection, your immune system releases hormones that temporarily raise blood sugar. This is a survival mechanism: your cells need extra fuel to mount an immune response. The effect applies to everything from a common cold to a urinary tract infection to the flu. People with diabetes often notice their glucose runs significantly higher during illness, sometimes requiring adjustments to their management plan. The spike typically resolves as the illness clears, but it can persist for days.
Medications That Raise Glucose
Several common medication classes can push blood sugar higher, sometimes enough to trigger a new diabetes diagnosis in someone who was previously in the normal range.
- Corticosteroids (prescribed for inflammation, asthma, autoimmune conditions) are the most well-known culprit. Oral forms carry the highest risk, but inhaled and even topical corticosteroids can raise glucose.
- Certain blood pressure medications contribute as well. Thiazide diuretics increase fasting glucose, partly by lowering potassium levels. Some beta blockers, particularly older non-selective types, are linked to new-onset diabetes.
- Statins for cholesterol carry a small but real increased risk of developing diabetes, with higher-potency versions showing greater risk.
- Antipsychotic medications, especially olanzapine and clozapine, are associated with both significant weight gain and elevated blood sugar.
- Some antibiotics, particularly fluoroquinolones, have been linked to blood sugar disruptions.
If you notice your glucose levels climbing after starting a new medication, that connection is worth discussing with your prescriber. In many cases, alternative medications with less metabolic impact are available.
Dehydration
When you don’t drink enough water, your blood sugar can rise through a less obvious pathway. Research on people with type 2 diabetes found that just three days of low water intake worsened their blood glucose response, and the mechanism involved cortisol. Dehydrated participants had persistently elevated cortisol levels, which as described above prompts the liver to produce more glucose. In well-hydrated participants, cortisol dropped normally after a glucose challenge. In the dehydrated group, it barely budged.
There’s also a simpler explanation at play: with less water in your bloodstream, the same amount of glucose becomes more concentrated. Staying well hydrated won’t cure high blood sugar, but chronic mild dehydration can make existing glucose control worse.
The Early Morning Spike
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, a natural hormonal shift in the early morning hours (roughly 3 a.m. to 8 a.m.) when the body releases hormones that counteract insulin. In people without diabetes, the pancreas compensates by producing more insulin. In people with diabetes, that compensation is insufficient, and glucose climbs.
A less common cause of morning highs is the Somogyi effect, which happens when too much insulin the evening before causes blood sugar to drop too low overnight. The body then overcorrects by flooding the bloodstream with glucose. The distinction matters because the solutions are opposite: the dawn phenomenon may require more insulin coverage in the early morning, while the Somogyi effect means reducing the evening dose. Checking blood sugar between 3 a.m. and 5 a.m. for a few nights, or using a continuous glucose monitor, can help sort out which is happening.
Caffeine’s Mixed Effects
Caffeine has a complicated relationship with blood sugar. In short-term studies, a dose equivalent to about two cups of coffee reduced insulin sensitivity in healthy young adults. Caffeine blocks receptors in muscle tissue that normally help with glucose uptake and also raises adrenaline levels, both of which can temporarily increase blood sugar after a meal.
Long-term coffee consumption, however, tells a different story. A meta-analysis found that habitual coffee drinking, including decaf, does not negatively affect insulin resistance or sensitivity over time. The current evidence suggests there’s no reason to restrict coffee intake specifically out of concern for blood sugar, whether you have diabetes, prediabetes, or normal glucose levels. The short-term spike from caffeine appears to be offset by other compounds in coffee that improve metabolic function over the long haul.
Alcohol’s Two-Phase Effect
Alcohol can affect blood sugar for up to 12 hours, and its impact goes in both directions. Mixed drinks, beer, and sweet wines contain carbohydrates that raise glucose initially. But as your liver processes the alcohol itself, it gets busy breaking down the alcohol and stops producing new glucose. This can cause blood sugar to drop, sometimes dangerously low, hours after drinking. The risk of delayed low blood sugar is highest when drinking on an empty stomach or after exercise, and it’s particularly relevant for anyone taking insulin or medications that stimulate insulin production.
Normal, Prediabetic, and Diabetic Ranges
Knowing what raises blood sugar is more useful when you understand the thresholds. According to the American Diabetes Association’s 2025 standards, a normal fasting blood sugar is below 100 mg/dL. Prediabetes is defined as a fasting glucose of 100 to 125 mg/dL, or an A1C of 5.7% to 6.4%. Diabetes is diagnosed at a fasting glucose of 126 mg/dL or higher, an A1C of 6.5% or higher, or a random glucose reading of 200 mg/dL or higher with symptoms like excessive thirst or frequent urination. A diagnosis in the absence of obvious symptoms requires two abnormal test results, either from different tests taken at the same time or the same test repeated on a separate day.