Low blood glucose, also called hypoglycemia, is when your blood sugar drops below 70 mg/dL. At that level, your body starts triggering warning signals to get sugar back into your bloodstream. While it’s most common in people with diabetes, it can happen to anyone under the right circumstances.
How Low Blood Sugar Is Classified
The American Diabetes Association breaks hypoglycemia into three levels based on severity. Level 1 is a blood sugar between 54 and 69 mg/dL. You’ll likely feel off, but you can treat it yourself. Level 2 is anything below 54 mg/dL, which is considered clinically significant and needs immediate attention. Level 3 isn’t defined by a specific number. It’s any episode severe enough to cause altered mental or physical functioning where you need someone else’s help to recover.
For context, a normal fasting blood sugar falls between 80 and 130 mg/dL before meals and stays below 180 mg/dL after eating.
What Happens Inside Your Body
Your brain runs almost entirely on glucose, so your body treats a falling blood sugar level as an emergency. Specialized neurons in the brain detect the drop and set off a chain reaction designed to push sugar back into your blood.
The first response kicks in around 64 to 74 mg/dL: your pancreas releases glucagon, a hormone that tells your liver to dump stored sugar into the bloodstream. One of the triggers for this release is simply that insulin production drops, which removes the brake on glucagon. If blood sugar keeps falling into the 55 to 68 mg/dL range, your body escalates by flooding your system with adrenaline and related stress hormones. These do several things at once: they push more sugar out of the liver, slow down sugar uptake by your muscles, and directly suppress insulin. This is why low blood sugar feels so physical. The shaking, sweating, and racing heart aren’t just symptoms. They’re your stress response working to save you.
Blocking this stress response in lab settings eliminates 75% to 90% of the glucagon response to low blood sugar, which shows just how central the nervous system is to the recovery process.
Symptoms to Recognize
Low blood sugar produces two distinct types of symptoms. The first set comes from your body’s adrenaline surge: sweating, shakiness, a racing heartbeat, anxiety, and sudden intense hunger. These tend to hit first and serve as early warnings.
The second set comes from your brain not getting enough fuel. These include weakness, fatigue, dizziness, trouble concentrating, confusion, and blurred vision. Some people behave in ways that look like intoxication, which can be dangerous if bystanders don’t realize what’s happening. In extreme cases, untreated low blood sugar leads to seizures or loss of consciousness.
How quickly these symptoms appear and how noticeable they are varies from person to person. People who experience frequent lows can develop something called impaired hypoglycemia awareness, where the early warning signs become muted. This is one reason the ADA recommends screening for it at least once a year in people at risk.
Low Blood Sugar During Sleep
Nocturnal hypoglycemia is particularly tricky because you can’t notice the usual warning signs while asleep. Signs that it’s happening include restless or irritable sleep, sweaty or clammy skin, trembling, sudden changes in breathing rate, nightmares, and a racing heartbeat.
Common triggers for nighttime drops include skipping dinner, exercising before bed, and drinking alcohol in the evening. Infections can also play a role. Certain types of insulin that peak six to eight hours after injection are particularly associated with nighttime lows when taken at dinner.
Common Causes
In people with diabetes, the most frequent cause is medication. Insulin, sulfonylureas, and meglitinides can all push blood sugar too low, especially if a meal is skipped or delayed, physical activity increases, or alcohol is consumed. The ADA recommends reevaluating the treatment plan after even a single episode of level 2 or level 3 hypoglycemia.
Hypoglycemia in people without diabetes is much less common but does occur. One recognized cause is reactive (or postprandial) hypoglycemia, where blood sugar drops within four hours of eating. This is most often seen in people who have had stomach surgery, particularly gastric bypass, which changes how quickly food moves through the digestive system. In children, insufficient growth hormone can be a cause.
How to Treat an Episode
The standard approach is called the 15-15 rule: eat 15 grams of fast-acting carbohydrate, then wait 15 minutes. If you still don’t feel better, eat another 15 grams. Any of these count as roughly 15 grams:
- 3 glucose tablets
- Half a cup (4 ounces) of fruit juice or regular soda
- 6 or 7 hard candies
- 1 tablespoon of sugar
Pure glucose is preferred over foods that contain fat or protein, because fat and protein slow digestion and delay the blood sugar rise when you need it fast. After treating, check your blood sugar to confirm it has returned to a safe range.
For severe episodes where someone is confused, unconscious, or unable to swallow safely, glucagon is the rescue treatment. The ADA recommends that everyone who takes insulin or is at high risk for hypoglycemia keep a glucagon kit accessible, and that family members and caregivers know where it is and how to use it. Newer formulations that don’t require mixing are preferred because they’re simpler to administer under stress.
Long-Term Risks of Repeated Lows
A single mild episode that’s quickly treated is not dangerous. Repeated or severe episodes are a different story. Because the brain depends on a constant supply of glucose, prolonged hypoglycemia can cause brain injury. Severe lows also trigger inflammatory and clotting changes in the blood, impair blood vessel function, reduce blood flow to the heart, and can cause dangerous heart rhythm disturbances.
In people with type 2 diabetes, severe hypoglycemia is one of the strongest predictors of major cardiovascular events and death. Large clinical trials have found that people who experience one or more severe episodes face a 1.7 to 4.3 times higher risk of dying compared to those who don’t. Studies estimate a mortality risk of 2% to 4% following a severe hypoglycemic episode in people with diabetes. These numbers underscore why preventing lows matters just as much as preventing highs in blood sugar management.