High and low blood sugar produce distinct sets of symptoms, though some overlap. Low blood sugar (hypoglycemia) typically hits fast with shakiness, sweating, and hunger, while high blood sugar (hyperglycemia) builds slowly with excessive thirst, frequent urination, and fatigue. The surest way to tell the difference is a quick fingerstick glucose test, but learning to read your body’s signals can help you act faster when your levels shift.
What the Numbers Mean
A normal blood sugar reading falls between 70 and 180 mg/dL for most of the day. Below 70 mg/dL is considered low, and anything above 180 mg/dL after meals or above 130 mg/dL when fasting is considered high. These thresholds matter because the symptoms you feel often don’t kick in until levels drift well past these cutoffs, especially if your body has gotten used to running high or low.
For people managing diabetes, the goal is to spend at least 70% of the day in that 70 to 180 range, with less than 4% of the day below 70 and less than 5% above 250.
Signs Your Blood Sugar Is Low
Low blood sugar tends to announce itself quickly because your brain depends on a steady glucose supply. The first wave of symptoms comes from your body’s stress response: sweating, shakiness, a racing heart, anxiety, and sudden intense hunger. These are your early warning signs, and they can appear within minutes of a drop.
If glucose keeps falling, a second set of symptoms takes over as your brain starts running short on fuel. You may feel weak, dizzy, or unusually tired. Concentration becomes difficult. Vision can blur. Some people behave oddly or seem confused, sometimes looking as though they’ve been drinking. In severe cases, seizures or loss of consciousness can follow.
Mood shifts are another clue. Low blood sugar is strongly linked to nervousness and anxiety. If you suddenly feel jittery or on edge for no clear reason, especially a couple of hours after eating, a glucose dip may be the cause.
Signs Your Blood Sugar Is High
High blood sugar is sneakier. It develops over hours or days rather than minutes, which makes it easier to miss. The classic trio of symptoms is excessive thirst, frequent urination, and unexplained weight loss. Your kidneys work overtime to flush the extra glucose, pulling water with it, which leaves you dehydrated and constantly thirsty.
You may also feel unusually tired, have blurry vision, or notice that cuts and scrapes heal more slowly than usual. Mood changes show up here too, but they look different from the nervousness of low blood sugar. High glucose levels are more often associated with irritability, anger, and sadness.
When blood sugar climbs dangerously high, the body can enter a state called diabetic ketoacidosis. Warning signs include nausea or vomiting, belly pain, shortness of breath, fruity-scented breath, and confusion. This is a medical emergency that requires immediate help.
Quick Comparison
- Onset: Low blood sugar hits in minutes. High blood sugar builds over hours or days.
- Hunger: Low blood sugar causes sudden, intense hunger. High blood sugar rarely does.
- Sweating and shakiness: Almost always low blood sugar, not high.
- Excessive thirst and frequent urination: Almost always high blood sugar.
- Confusion: Can happen with either extreme, but with low blood sugar it appears alongside shakiness, while with high blood sugar it appears alongside dehydration and nausea.
- Mood: Low blood sugar brings nervousness. High blood sugar brings irritability or sadness.
Why Symptoms Alone Aren’t Always Reliable
Some people, particularly those who have had type 1 diabetes for years, lose the ability to feel low blood sugar before it becomes dangerous. This condition, called hypoglycemia unawareness, is surprisingly common. Studies have found that roughly half to two thirds of people with type 1 diabetes have a diminished ability to sense dropping glucose levels. When early warning symptoms like sweating and shakiness fail to appear, the first sign of trouble may be confusion or passing out. People with hypoglycemia unawareness face a three to six-fold higher risk of severe low blood sugar episodes.
On the other end, people who have been running high for a long time may feel “normal” at elevated levels and actually experience symptoms of low blood sugar when their numbers drop into a healthy range. This is temporary, as the body readjusts, but it can make symptoms misleading during the transition.
How to Check for Certain
A fingerstick glucose meter gives you a result from a tiny drop of blood in seconds. It measures glucose directly from your capillaries and is the most straightforward way to confirm what your symptoms are telling you. If you feel off and aren’t sure whether you’re high or low, checking with a meter removes the guesswork entirely.
Continuous glucose monitors (CGMs) take a reading every few minutes from the fluid just under your skin. They’re useful for spotting trends and catching highs or lows overnight. One thing to keep in mind: there’s a 5 to 20 minute lag between your blood glucose and the interstitial fluid a CGM measures. During rapid swings, such as after a meal or during exercise, the CGM number may not match what a fingerstick would show. If a CGM reading doesn’t match how you feel, a fingerstick is the better tie-breaker.
How often you need to check depends on your situation. People with type 1 diabetes often test 4 to 10 times a day: before meals and snacks, at bedtime, sometimes overnight, and after treating a low. People with type 2 diabetes who use insulin may test several times daily or use a CGM. If you don’t take insulin, your doctor may recommend less frequent checks, often focused on fasting readings and post-meal checks when adjusting diet or medication.
Low Blood Sugar Without Diabetes
You don’t need to have diabetes to experience low blood sugar. Reactive hypoglycemia causes a glucose drop in the two to four hours after eating, often after a meal high in refined carbohydrates. The body overproduces insulin in response to the meal, which then drives blood sugar below normal. Symptoms are the same: shakiness, sweating, hunger, and lightheadedness.
People who have had gastric bypass surgery are especially prone to post-meal lows because food enters the small intestine faster, triggering a rapid and oversized insulin response. Other causes of non-diabetic hypoglycemia include prolonged fasting, intense exercise without adequate fuel, and certain medications or medical conditions that affect insulin production.
What to Do When You Confirm a Low
The standard approach for treating confirmed low blood sugar is called the 15-15 rule. Eat or drink 15 grams of fast-acting carbohydrates, such as four glucose tablets, half a cup of juice, or a tablespoon of honey. Wait 15 minutes and recheck. If you’re still below 70 mg/dL, repeat with another 15 grams. Keep going until your level is back in your target range.
The key is using simple sugars that absorb quickly. A candy bar or cookie has fat that slows digestion, so it won’t bring your number up as fast as pure glucose or juice will. Once your blood sugar stabilizes, follow up with a small snack that includes protein or complex carbs to keep it from dropping again.
What to Do When You Confirm a High
A single high reading after a big meal isn’t necessarily cause for alarm. Persistent highs, readings above 180 mg/dL that don’t come down within a few hours, need attention. If you use insulin, your treatment plan likely includes correction doses for these situations. Physical activity can also help bring levels down, though you should avoid exercise if your blood sugar is above 250 mg/dL and you’re showing signs of ketoacidosis, since exercise can push levels higher in that state.
Staying hydrated helps your kidneys clear excess glucose. If high readings persist despite your usual management steps, or if you develop nausea, vomiting, abdominal pain, or fruity-smelling breath, you need emergency medical care. These are signs that the situation has moved beyond what you can manage at home.