Hypoglycemia is low blood sugar, and hyperglycemia is high blood sugar. The dividing line: blood sugar below 70 mg/dL is considered hypoglycemic, while levels consistently above 180 mg/dL after meals cross into hyperglycemic territory. Both can happen to people with diabetes, and both can become dangerous, but they feel different, have different causes, and require opposite responses.
The Numbers That Define Each
Normal blood sugar for most adults with diabetes falls between 80 and 130 mg/dL before meals and stays below 180 mg/dL after eating. Anything below or above those boundaries starts to cause problems.
Hypoglycemia has two clinical levels. Level 1 is blood sugar between 54 and 69 mg/dL, a range where you’ll likely feel symptoms and can treat it yourself. Level 2 is below 54 mg/dL, which is more serious and can impair your ability to think clearly or function. Level 3 is any episode where you need someone else’s help to recover, regardless of the exact number.
Hyperglycemia doesn’t have the same tiered system for everyday highs, but the emergency thresholds are well defined. Diabetic ketoacidosis (DKA) involves blood sugar at or above 200 mg/dL combined with a dangerous buildup of acids in the blood. Hyperglycemic hyperosmolar state, a rarer emergency, involves blood sugar at or above 600 mg/dL and can lead to coma.
How They Feel Different
Low blood sugar hits fast and feels urgent. Your body floods with stress hormones, which is why the symptoms feel so physical: a racing pulse, cold sweats, shaking, pale skin, and sudden intense hunger. You might feel anxious or restless, have trouble concentrating, or get a headache. The experience often comes on within minutes.
High blood sugar is more of a slow burn. The classic signs are extreme thirst and frequent urination, because your kidneys are working overtime to flush excess sugar. You’ll feel tired, listless, possibly nauseated or dizzy. These symptoms can develop over hours or days, which makes them easier to miss or attribute to something else.
Both extremes can lead to confusion and loss of consciousness if they get severe enough. That overlap is why checking your actual blood sugar number matters so much. Feeling “off” alone won’t tell you which direction the problem is going.
What Causes Each One
Hypoglycemia typically results from too much insulin relative to the sugar in your blood. That can happen if you take too much diabetes medication, skip a meal, eat less than planned, drink alcohol, or exercise harder than usual without adjusting your food or medication. It’s most common in people taking insulin or certain oral diabetes drugs that stimulate insulin production.
Hyperglycemia happens when there isn’t enough insulin to move sugar from your blood into your cells. The triggers are essentially the mirror image: eating more carbohydrates than your medication can cover, skipping doses, taking medication incorrectly, being less physically active, or dealing with illness or stress. Infections and certain medications like steroids can also push blood sugar up.
What to Do When It Happens
For low blood sugar, the standard approach is the 15-15 rule. Eat 15 grams of fast-acting carbohydrate, then wait 15 minutes and recheck. Fifteen grams looks like three glucose tablets, half a cup of juice or regular soda, six or seven hard candies, or one tablespoon of sugar. If your level hasn’t come up after 15 minutes, repeat. Once it normalizes, eat a small snack or meal to keep it stable.
For high blood sugar, the response depends on your treatment plan. Physical activity can help bring levels down in many cases, as can drinking water to stay hydrated. If you use insulin, your care team will have given you guidance on correction doses. Persistent highs above 240 mg/dL, especially with symptoms like nausea or vomiting, need more urgent attention because they can signal DKA.
Long-Term Risks of Each
Chronic hyperglycemia is the driving force behind most diabetes complications. Persistently elevated blood sugar damages blood vessels and nerves over months and years, increasing the risk of heart disease, kidney damage, vision loss, and nerve problems in the feet and hands. The American Diabetes Association recommends keeping A1C (a measure of average blood sugar over roughly three months) below 7% for most adults, which translates to an average blood sugar around 154 mg/dL.
Repeated hypoglycemia carries its own serious risks. Severe episodes involving seizures or loss of consciousness are associated with increased mortality. Perhaps the most dangerous long-term consequence is a condition called hypoglycemia unawareness, where your body stops producing the normal warning signs of low blood sugar. The stress hormones that cause sweating, shaking, and a racing heart gradually become muted after repeated lows. Without those alarms, you can drop into dangerously low territory without realizing it, creating a cycle: unrecognized lows lead to more severe episodes, which further blunt your body’s warning system. Risk factors include long duration of diabetes, tight blood sugar control, prior severe episodes, alcohol use, and kidney problems.
Morning Blood Sugar Spikes
If you wake up with unexpectedly high blood sugar, two distinct patterns could be responsible. The more common one is the dawn phenomenon, where your body’s natural hormones (which counteract insulin) surge in the early morning hours while your insulin levels are at their lowest. This pushes blood sugar up before you’ve eaten anything.
The less common pattern, called the Somogyi effect, is actually triggered by low blood sugar during the night. If your evening insulin dose is too high, your blood sugar drops while you sleep, and your body overcompensates by releasing stored sugar, leaving you high by morning. The distinction matters because the treatments are opposite: the dawn phenomenon may call for more insulin coverage overnight, while the Somogyi effect means reducing it. Checking blood sugar between 3 and 5 a.m. for a few nights, or using a continuous glucose monitor, can reveal which pattern is happening.
Why the Difference Matters
Managing diabetes is essentially a balancing act between these two states. Tightening blood sugar control to prevent the long-term damage of hyperglycemia increases the risk of hypoglycemia. Loosening control to avoid dangerous lows lets blood sugar run higher. The American Diabetes Association acknowledges this tradeoff directly: for people who experience frequent or severe lows, less stringent targets may be appropriate, while people with low risk of hypoglycemia may benefit from tighter goals like an A1C below 6.5%.
Knowing the symptoms of both, keeping fast-acting sugar accessible, and understanding your own patterns are the practical tools that make this balance manageable day to day.