High blood sugar happens when glucose builds up in your bloodstream instead of being absorbed by your cells. A fasting blood sugar of 126 mg/dL or higher signals diabetes, while a random reading of 200 mg/dL or higher at any time of day does the same. But blood sugar can spike well above normal for reasons that go far beyond diabetes, including stress, poor sleep, medications, and what you ate for lunch.
Type 1 and Type 2 Diabetes
The two most common forms of diabetes cause high blood sugar through different mechanisms. In type 1 diabetes, the immune system attacks and destroys the insulin-producing cells in the pancreas. Without insulin, glucose has no way to enter your cells and accumulates in the blood instead.
Type 2 diabetes starts with insulin resistance. Your muscle, fat, and liver cells stop responding to insulin the way they should, so your body demands more and more of it to keep blood sugar in check. Over time, the pancreas can’t keep up with that demand, and blood sugar rises. Insulin resistance is the central driver of type 2 diabetes and can develop years before blood sugar levels become abnormal enough for a diagnosis.
How Food Affects Blood Sugar
All carbohydrates raise blood sugar, but not equally. The glycemic index ranks foods on a scale of 0 to 100 based on how fast they spike your blood glucose, with pure sugar scoring 100. Processed foods generally score higher, while foods rich in fiber or fat score lower because they slow digestion and the release of glucose into your bloodstream.
The glycemic index alone doesn’t tell the full story, though. A food’s glycemic load accounts for both the speed of the blood sugar rise and the amount of carbohydrate in a typical serving. Watermelon has a high glycemic index of 80, but a normal serving contains so little carbohydrate that its glycemic load is only 5. A bowl of white rice, by contrast, delivers both a fast spike and a large dose of carbohydrate. When trying to understand which foods are pushing your blood sugar up, glycemic load is the more useful number.
Stress and the Cortisol Response
Physical or emotional stress triggers a hormonal cascade designed to flood your body with quick energy. Insulin levels drop while adrenaline, cortisol, glucagon, and growth hormone all rise. This combination tells your liver to dump stored glucose into the bloodstream. At the same time, cortisol and growth hormone make your muscle and fat tissue less sensitive to insulin, so that glucose stays in circulation longer. The result is a noticeable blood sugar spike even if you haven’t eaten anything.
This response evolved to fuel a fight-or-flight situation, but it doesn’t distinguish between a physical threat and a stressful work deadline. Chronic stress keeps cortisol elevated for extended periods, which can push blood sugar higher day after day.
Illness and Infection
Getting sick, whether it’s a cold, the flu, or a urinary tract infection, commonly raises blood sugar. Your immune system releases hormones to help fight off the illness, and those same hormones increase blood glucose temporarily. For people with diabetes, this can make blood sugar much harder to control during even a mild illness. For people without diabetes, the spike is usually modest and resolves as the infection clears.
Sleep Deprivation
Cutting just 1.5 hours off your normal sleep time can measurably change how your body handles sugar. An NIH-funded study found that women who slept 6.2 hours per night instead of their usual 7.5 hours developed a 14.8% increase in insulin resistance over six weeks. Postmenopausal women were hit harder, with insulin resistance climbing as much as 20.1%. Importantly, these changes were not explained by weight gain. The women didn’t eat significantly more or put on meaningful weight during the study. Their cells simply stopped responding to insulin as well.
The encouraging finding: once the women returned to sleeping 7 to 9 hours per night, their insulin and glucose levels went back to normal. This suggests that chronic mild sleep loss is a reversible but underappreciated cause of elevated blood sugar.
Medications That Raise Blood Sugar
Several common medications can push blood sugar up as a side effect, even in people who don’t have diabetes:
- Corticosteroids (prescribed for inflammation, asthma, autoimmune conditions) are among the most potent offenders, directly increasing insulin resistance and triggering glucose release from the liver.
- Thiazide diuretics, often used for high blood pressure, can raise fasting glucose.
- Birth control pills containing estrogen or progesterone may modestly affect blood sugar regulation.
- Decongestants like pseudoephedrine, found in many over-the-counter cold medicines.
- Some antipsychotic medications can cause significant metabolic changes, including weight gain and insulin resistance.
- High-dose niacin (vitamin B3), though the blood sugar effect tends to diminish after a few months of use.
If you’ve noticed your blood sugar creeping up after starting a new medication, that connection is worth discussing at your next appointment.
Other Medical Conditions
Several conditions can cause high blood sugar independent of type 1 or type 2 diabetes. Cushing’s syndrome, which involves chronic overproduction of cortisol, directly impairs insulin function. Pancreatitis and cystic fibrosis can damage the pancreas enough to reduce insulin production. These situations are sometimes grouped under the term “secondary diabetes” because the elevated blood sugar is a consequence of another underlying problem.
Morning Blood Sugar Spikes
Many people notice their blood sugar is highest first thing in the morning, even before eating. The most common explanation is the dawn phenomenon: between roughly 4 and 8 a.m., your body naturally releases cortisol and growth hormone to prepare you for waking. These hormones raise blood sugar as part of your normal circadian rhythm. The dawn phenomenon is extremely common and doesn’t require a nighttime low to trigger it.
A less common cause is the Somogyi effect, or rebound hyperglycemia. This happens specifically in people who take insulin. If injected insulin causes blood sugar to drop too low during the night, the body panics and releases a surge of adrenaline, glucagon, cortisol, and growth hormone. These hormones tell the liver to release large amounts of stored glucose, overcorrecting the low and producing a high reading by morning. The key difference: the Somogyi effect starts with a nighttime low, while the dawn phenomenon does not.
Physical Inactivity
Your muscles are the largest consumers of blood glucose in your body, especially during movement. When you’re sedentary for long stretches, your muscles take up less glucose, and insulin sensitivity gradually declines. Even short periods of inactivity, like sitting for several hours after a meal, allow blood sugar to stay elevated longer than it would if you took a brief walk. This is one reason why post-meal blood sugar readings can vary dramatically depending on whether you sit on the couch or move around after eating.