High blood sugar often announces itself through a handful of recognizable symptoms, but it can also run silently in the background with no obvious signs at all. The most reliable way to know is to test with a blood glucose meter, where a fasting reading above 130 mg/dL or a reading above 180 mg/dL two hours after eating generally signals a problem. But your body also sends physical signals worth learning to recognize.
The Earliest Physical Signs
The two hallmark symptoms of high blood sugar are excessive thirst and frequent urination, and they’re directly connected. When glucose builds up in your bloodstream beyond what your kidneys can reabsorb, the excess sugar spills into your urine. It pulls extra water along with it through a process called osmotic diuresis. The result: you urinate more than usual, lose fluid, and feel intensely thirsty as your body tries to compensate.
This cycle can start subtly. You might notice you’re refilling your water bottle more often, waking up at night to use the bathroom, or feeling thirsty despite drinking plenty of fluids. Other early signs include increased hunger (even shortly after eating), headaches, and blurred vision. As high blood sugar persists over days or weeks, fatigue becomes a dominant symptom because your cells aren’t efficiently converting glucose into energy.
Why You Might Have No Symptoms at All
Some people walk around with elevated blood sugar for months or years without feeling anything unusual. This is especially common in people who have had type 2 diabetes for a long time. Their bodies gradually adjust to higher glucose levels, and the classic warning signs fade or never appear in the first place. This is one of the reasons type 2 diabetes often goes undiagnosed.
The absence of symptoms doesn’t mean the damage stops. Sustained high blood sugar, even when you feel fine, can quietly injure blood vessels and nerves throughout the body. Over time this leads to complications affecting the eyes (retinal damage that can cause blindness), kidneys (progressive damage leading to failure), nerves in the feet and hands (pain, numbness, and infection risk), and the cardiovascular system. The only way to catch silent high blood sugar is through regular testing.
How to Test at Home
A blood glucose meter (glucometer) gives you a number in about five seconds. The process is straightforward: you insert a test strip into the meter, prick the side of a fingertip with a lancet device, touch the drop of blood to the strip, and read the result. Most lancet devices have an adjustable depth dial so you can find the shallowest setting that still draws enough blood. Wash your hands with warm water beforehand, since residue from food or lotion on your fingers can skew the reading.
A few tips that make testing more accurate: take the test strip out of the container and set it on top of your meter to keep it clean and dry until you’re ready. Close the container tightly afterward, because moisture damages unused strips. Dispose of used lancets in a hard plastic container with a lid, not directly in the trash.
What the Numbers Mean
For people under 60 with no other medical conditions, a healthy fasting blood sugar typically falls between 80 and 120 mg/dL. For people 60 and older, a range of 100 to 140 mg/dL is considered reasonable. Before meals, the general target is 80 to 130 mg/dL. Two hours after eating, blood sugar should be below 180 mg/dL.
If your meter reads 240 mg/dL or higher, it’s worth testing your urine for ketones using over-the-counter test strips. Ketones are acids your body produces when it starts burning fat instead of glucose for fuel, and high levels can signal a dangerous condition called diabetic ketoacidosis. This is most relevant for people with type 1 diabetes, but anyone seeing numbers that high should pay close attention.
Emergency Warning Signs
When blood sugar climbs very high and stays there, the body can tip into a medical crisis. Diabetic ketoacidosis produces a distinct set of symptoms that escalate quickly: fast, deep breathing; dry skin and mouth; a flushed face; nausea and vomiting; stomach pain; muscle stiffness; and a fruity or acetone-like smell on the breath. That fruity breath is one of the most specific red flags, caused by ketones being exhaled through the lungs.
Get emergency care if your blood sugar stays at 300 mg/dL or above, your breath smells fruity, you’re vomiting and can’t keep food or liquids down, or you’re having difficulty breathing. Left untreated, these conditions can progress to diabetic coma.
Surprising Things That Raise Blood Sugar
Food and missed medication are the obvious culprits, but several less expected factors can push your numbers up. Even a single night of poor sleep reduces how effectively your body uses insulin. Dehydration concentrates the sugar already in your blood, making readings climb without you eating anything at all. Sunburn triggers a stress response that raises blood sugar. Certain nasal decongestant sprays contain chemicals that signal your liver to release more glucose. Even gum disease has been linked to higher blood sugar levels.
There’s also the dawn phenomenon, a natural hormone surge that happens in the early morning hours. Everyone experiences it, but for people with diabetes it can cause a noticeable blood sugar spike before breakfast, which is why fasting morning readings sometimes seem unexpectedly high even when you ate well the night before.
Patterns Matter More Than Single Readings
A single high reading isn’t necessarily cause for alarm. Stress, a carb-heavy meal, or poor sleep the night before can produce a temporary spike that resolves on its own. What matters more is the pattern. If your fasting readings are consistently above 130 mg/dL, or your post-meal numbers regularly exceed 180 mg/dL, that trend points to a blood sugar regulation problem worth investigating.
Keeping a simple log of your readings, along with what you ate and how you slept, helps you spot those patterns. It also gives your healthcare provider far more useful information than a single lab draw taken on one particular morning. Many meters store readings automatically, and some sync to phone apps that track trends over weeks and months.