Blood sugar can spike for dozens of reasons beyond the obvious ones like eating a candy bar or drinking soda. Stress, poor sleep, dehydration, medications, and even the time of day all play a role. If you’re seeing unexpectedly high readings on your glucose monitor, the cause is often something you wouldn’t think to blame.
Stress and the Hormonal Cascade
Stress is one of the most common and most overlooked drivers of high blood sugar. When your body perceives a threat, whether physical or emotional, it shifts into a survival mode that prioritizes having energy available in your bloodstream. Insulin levels drop, adrenaline and cortisol rise, and your liver dumps stored glucose into your blood. At the same time, cortisol and growth hormone make your muscles and fat tissue less responsive to insulin, so that glucose stays in circulation longer instead of being absorbed by cells.
This means anything that stresses your body can raise your blood sugar: a bad day at work, a sunburn, chronic pain, an argument, or even a scary movie. The effect isn’t subtle. For people with diabetes, a stressful event can push readings well above their target range without them eating a single thing.
Sleep, Breakfast, and Daily Timing
Even one night of poor sleep makes your body use insulin less efficiently. If you’ve been running on five or six hours, your morning reading may be higher than expected simply because your cells aren’t responding to insulin the way they normally would.
Skipping breakfast compounds the problem. Going without a morning meal has been shown to increase blood sugar after both lunch and dinner, likely because your body stays in a fasting, stress-hormone-driven state longer than it should. Blood sugar also tends to be harder to control later in the day, so the combination of missed sleep and a skipped breakfast can set off a chain reaction that keeps your levels elevated well into the evening.
The Dawn Phenomenon
If your blood sugar is specifically high first thing in the morning, the most likely explanation is something called the dawn phenomenon. In the early morning hours, your body naturally releases cortisol and growth hormone to prepare you for waking up. These hormones trigger glucose release from the liver and reduce insulin sensitivity. Everyone experiences this hormonal surge, but in people with diabetes or prediabetes, the body can’t compensate with enough insulin to keep glucose in range.
A less common cause of high morning sugar is what’s known as the Somogyi effect. This happens when blood sugar drops too low overnight, usually because of insulin taken before bed. Your body panics and floods the bloodstream with adrenaline, glucagon, cortisol, and growth hormone, all of which push your liver to release large amounts of stored glucose. You wake up with a high reading that looks like the dawn phenomenon but actually started with a low. If you suspect this, checking your blood sugar around 2 or 3 a.m. for a few nights can help you tell the difference.
Illness and Infection
When your immune system fights off an illness, it releases hormones that temporarily raise blood sugar. This happens with everything from a cold to a urinary tract infection to a stomach bug. The sicker you are, the higher your sugar is likely to go. For people with diabetes, even a mild infection can push readings significantly above normal, and the effect can last for the entire duration of the illness.
This is worth paying attention to because persistently high blood sugar during illness raises the risk of a dangerous complication called diabetic ketoacidosis, or DKA. Early signs include extreme thirst and urinating much more than usual. More severe symptoms can develop quickly: fast, deep breathing, fruity-smelling breath, nausea, vomiting, and stomach pain. If your blood sugar stays at 300 mg/dL or above, your breath smells fruity, or you can’t keep fluids down, that’s a medical emergency.
Dehydration
When you’re dehydrated, the total amount of glucose in your blood doesn’t change, but the water volume carrying it decreases. The result is a higher concentration of sugar per unit of blood, which shows up as an elevated reading on your meter. Think of it like adding the same amount of sugar to a half-full glass of water versus a full glass. The half-full glass tastes sweeter even though the sugar content is identical.
This means something as simple as not drinking enough water on a hot day, sweating heavily during exercise, or being sick with diarrhea or vomiting can produce a misleadingly high blood sugar reading. Rehydrating often brings the number back down without any other intervention.
Medications You Might Not Suspect
Several common medications raise blood sugar as a side effect. Corticosteroids are the biggest offenders. These are prescribed for conditions like asthma, arthritis, and allergic reactions, and they can dramatically increase glucose levels even in people without diabetes. But steroids aren’t limited to pills. Steroid-containing eye drops, nasal sprays, and even some sinus medications can trigger a spike.
Other medications that can nudge blood sugar upward include certain blood pressure drugs (beta-blockers), niacin (vitamin B3, often prescribed for cholesterol), some antidepressants, and certain antibiotics. If you’ve recently started a new medication and noticed your readings climbing, the prescription could be the explanation.
Exercise That Backfires
Moderate aerobic exercise like walking or cycling generally lowers blood sugar. But intense exercise can do the opposite. Strength training and high-intensity interval training both trigger a short-term spike in blood sugar because your body releases stress hormones to fuel the effort. Your liver responds by dumping glucose into the bloodstream faster than your muscles can absorb it.
This spike is usually temporary and resolves within an hour or two. Over time, regular exercise improves insulin sensitivity and lowers blood sugar overall. But if you check your reading immediately after a hard workout and see a number higher than you expected, the exercise itself is the most likely cause.
Foods That Surprise You
The obvious culprits, sugary drinks, white bread, pastries, are well known. But some foods raise blood sugar in ways you might not anticipate. Very large servings of protein (over 75 grams in a single meal) can cause a delayed blood sugar rise three to five hours later, even without many carbohydrates. This happens because your liver can convert amino acids from protein into glucose through a process that unfolds slowly.
Caffeine is another surprise. Even black coffee, with no sugar or cream, can raise blood sugar in people whose bodies are sensitive to caffeine. Some artificial sweeteners may also cause a spike, though the research on this is still evolving. And foods marketed as “sugar-free” or “healthy” often contain refined carbohydrates or starches that convert to glucose just as quickly as table sugar.
When High Blood Sugar Becomes Dangerous
An occasional reading above your target range is common and usually not an emergency. But sustained high blood sugar or very high individual readings require attention. If your blood sugar is 250 mg/dL or above, you should check it every four to six hours and test your urine for ketones if you have diabetes.
The red flags that call for immediate emergency care: blood sugar that stays at 300 mg/dL or above, fruity-smelling breath, vomiting that prevents you from keeping food or fluids down, trouble breathing, or a combination of extreme thirst, rapid breathing, and stomach pain. These are signs of DKA, which can become life-threatening within hours if untreated.