Blood sugar rises whenever glucose enters your bloodstream faster than your body can clear it. The most obvious trigger is eating, especially carbohydrate-rich foods, but stress, poor sleep, illness, certain medications, and even your morning wake-up hormones can push glucose levels higher. Understanding these causes helps explain why blood sugar sometimes spikes in situations that seem to have nothing to do with food.
How Food Raises Blood Sugar
Carbohydrates are the primary dietary driver of blood sugar. When you eat bread, rice, fruit, or anything containing starch or sugar, your digestive system breaks those carbohydrates down into simple sugars, mainly glucose. Enzymes in your saliva and small intestine split complex starches into smaller fragments, and a second round of enzymes along the intestinal wall finishes the job. The resulting glucose molecules are actively transported through the intestinal lining and into your bloodstream.
How fast this happens depends on the type of carbohydrate. Simple sugars in soda, candy, or fruit juice need very little processing and hit your bloodstream quickly. Complex carbohydrates in whole grains or legumes take longer to disassemble, producing a slower, more gradual rise. Fiber, protein, and fat all slow digestion further. Protein-rich foods like eggs, chicken, or nuts take three to four hours to digest, and fat slows the process even more. That’s why eating carbohydrates alongside protein, fat, or fiber blunts the glucose spike compared to eating carbs alone. A piece of white bread on its own will raise your blood sugar faster and higher than the same bread eaten with peanut butter.
For reference, a normal fasting blood sugar (after at least eight hours without eating) is below 100 mg/dL. Two hours after a meal, a healthy reading stays below 140 mg/dL. Fasting levels of 100 to 125 mg/dL fall in the prediabetes range, and 126 mg/dL or higher on two separate tests indicates diabetes.
Stress Hormones and the Liver
Your liver stores glucose in a compact form and releases it when your body needs quick energy. Stress hormones are one of the main signals that trigger this release, which is why blood sugar can climb even when you haven’t eaten anything.
Adrenaline acts directly on the liver to promote glucose production. It also triggers the breakdown and release of fat, which travels to the liver and gets converted into additional glucose and ketones. This is the classic fight-or-flight response: your body floods the bloodstream with fuel in case you need to run or react. Cortisol, the other major stress hormone, works differently. It makes fat and muscle cells resistant to insulin, meaning those tissues absorb less glucose from the blood. At the same time, cortisol ramps up glucose production in the liver. The combination, more glucose coming in and less being cleared, pushes blood sugar up.
This matters beyond moments of acute fear or danger. Chronic psychological stress, demanding work schedules, and ongoing anxiety all keep cortisol elevated for extended periods, creating a persistent upward pressure on blood sugar that many people never connect to their glucose readings.
Why Blood Sugar Rises Overnight
If you’ve ever checked your blood sugar first thing in the morning and found it higher than when you went to bed, you’ve experienced what’s called the dawn phenomenon. Between roughly 3 a.m. and 8 a.m., your body releases a surge of cortisol and growth hormone to prepare you for waking. These hormones signal the liver to produce more glucose, giving you the energy to start the day. In people with diabetes or insulin resistance, the body can’t compensate for this surge effectively, and fasting glucose ends up elevated.
A related but distinct pattern is the Somogyi effect. This happens when blood sugar drops too low during the night, often because of a missed meal or too much evening insulin. The body overcompensates by dumping glucose from the liver, and you wake up with high readings. The difference matters because the solutions are opposite: the dawn phenomenon may call for adjustments to evening medication timing, while the Somogyi effect means you’re getting too much insulin or not enough food before bed.
Illness and Infection
Getting sick almost always raises blood sugar, even if you’re eating less than usual. When your immune system fights an infection, it releases inflammatory signaling molecules that interfere with insulin’s ability to work. One key inflammatory signal impairs insulin receptor function in fat tissue and muscle, reducing glucose uptake and promoting insulin resistance. Another has been shown to directly induce higher blood sugar and force the body to produce extra insulin to compensate.
This creates a feedback loop. Higher blood sugar itself triggers more inflammatory signaling through a process involving oxidative stress, which in turn worsens insulin resistance. It’s one reason people with diabetes are told to monitor their glucose more carefully during any illness, from a common cold to a urinary tract infection. The blood sugar disruption can persist for days after other symptoms have resolved.
Exercise: It Depends on the Type
Moderate aerobic exercise, like walking or cycling at a conversational pace, typically lowers blood sugar by helping muscles absorb glucose without needing as much insulin. But not all exercise works this way. Strength training and high-intensity interval training can temporarily raise blood sugar. During intense bursts of effort, your body perceives the exertion as a form of stress and releases adrenaline, which triggers the liver to push out stored glucose. For most people this spike is short-lived and followed by improved glucose levels over the next several hours. But if you check your blood sugar immediately after a hard lifting session or sprint workout, don’t be surprised to see it higher than before you started.
Medications That Raise Blood Sugar
Corticosteroids, commonly prescribed for conditions like asthma, arthritis, and autoimmune disorders, are among the most significant medication-related causes of elevated blood sugar. These drugs increase glucose production in the liver by boosting the activity of enzymes involved in making new glucose. They also promote the storage of visceral fat, which releases fatty acids and signaling molecules that worsen insulin resistance. On top of that, corticosteroids directly counteract insulin’s effects on muscle and fat cells, making it harder for your body to clear glucose from the blood.
The effect can be substantial, and it isn’t limited to people who already have diabetes. Anyone on a course of steroids, whether oral, injected, or even high-dose inhaled, can experience significant blood sugar elevations. Other medications that may raise glucose include certain blood pressure drugs, antipsychotics, and some immunosuppressants.
Sleep Deprivation
Even a single night of poor sleep measurably affects blood sugar regulation. Research published in Diabetes Care found that one night of partial sleep restriction reduced insulin sensitivity by 14 to 21 percent. That means the body needed substantially more insulin to move the same amount of glucose out of the bloodstream. The glucose disposal rate, a measure of how efficiently muscles take up sugar, dropped by about 14 percent after a short night compared to a full one.
Over time, chronic short sleep compounds this effect. If you regularly sleep fewer than six hours, your body operates in a state of mildly impaired insulin function day after day. This is one of the less obvious but more impactful contributors to rising blood sugar, particularly for people who feel they’re eating well and exercising but still see higher-than-expected glucose readings.
Dehydration
When you’re dehydrated, blood sugar can rise through a mechanism that has nothing to do with food. Reduced blood volume activates a hormonal cascade called the renin-angiotensin-aldosterone system. Elevated aldosterone, one of the hormones in this cascade, has been shown to interrupt normal insulin signaling, slowing the removal of glucose from the bloodstream. Earlier in the same cascade, increased renin activity generates reactive oxygen species that further disrupt how insulin works. The result is that even mild dehydration can worsen glucose control, particularly in people who already have type 2 diabetes. Staying well-hydrated won’t cure high blood sugar, but chronic under-drinking can quietly make it worse.