Low blood sugar, called hypoglycemia, is generally defined as a blood glucose level below 70 mg/dL (3.9 mmol/L). In people without diabetes, blood sugar dropping below 60 mg/dL is typically considered the clinical threshold. The severity of symptoms depends on how far your levels drop, how quickly they fall, and whether your body has learned to recognize the warning signs.
The Three Levels of Low Blood Sugar
The American Diabetes Association classifies hypoglycemia into three levels based on glucose readings and how your body responds. These categories help determine how urgently you need to act.
Level 1 is a glucose reading between 54 and 69 mg/dL (3.0 to 3.9 mmol/L). At this stage, most people notice early warning signs: shakiness, hunger, sweating, a fast or irregular heartbeat, and irritability. These symptoms are your body’s alert system working as intended, triggered by a surge of stress hormones that signal your brain to find fuel.
Level 2 begins below 54 mg/dL (3.0 mmol/L). This is the point where your brain starts running short on glucose, its primary energy source. Symptoms shift from uncomfortable to concerning: confusion, slurred speech, blurry vision, difficulty completing routine tasks, and loss of coordination. Level 2 requires immediate action.
Level 3 is defined not by a specific number but by what happens to you. It means your mental or physical functioning is so impaired that you need someone else to help you recover. This can include loss of consciousness and seizures.
What Causes Blood Sugar to Drop
For people with diabetes, the most common cause is too much insulin or other glucose-lowering medication relative to the amount of food eaten or physical activity performed. Skipping a meal, eating less than usual, or exercising more intensely than planned can all tip the balance.
People without diabetes can also experience low blood sugar, though it’s less common. This falls into two broad categories based on timing. Reactive hypoglycemia (also called postprandial hypoglycemia) happens within four hours of eating a meal. In many cases, the exact cause isn’t clear, though it’s often connected to what and when you eat. Alcohol, previous gastric bypass surgery, inherited metabolic conditions, and certain rare tumors can also trigger it.
Fasting hypoglycemia occurs when you haven’t eaten for an extended period. It can be related to medications, liver disease, hormone deficiencies, or excessive alcohol consumption. If you’re experiencing repeated episodes of low blood sugar without an obvious explanation, that pattern itself is worth investigating.
Low Blood Sugar During Sleep
Nocturnal hypoglycemia is particularly tricky because you can’t recognize the warning signs while you’re asleep. Blood sugar dropping below 70 mg/dL during the night can cause restless sleep, night sweats, trembling, nightmares, changes in breathing patterns, and a racing heartbeat. A bed partner may notice these signs before you do.
Clues that it happened overnight include waking up with a headache, feeling unusually tired despite a full night’s sleep, or finding damp sheets. If this is a recurring problem, setting an alarm to check blood sugar in the early morning hours (typically between 2 and 4 a.m.) can help identify how often episodes occur. A continuous glucose monitor can also catch these drops automatically and wake you with an alert.
Why Some People Stop Feeling Symptoms
Repeated episodes of low blood sugar can dull your body’s alarm system over time. This is called hypoglycemia unawareness, and it’s most common in people with type 1 diabetes or those who have had diabetes for many years. Your body essentially recalibrates, suppressing the stress hormone response that normally produces shakiness and sweating. The result is that blood sugar can drop to dangerous levels without any obvious warning.
One clinical marker that suggests unawareness is hypoglycemic episodes lasting an average of about 107 minutes or longer, which indicates you’re spending significant time at low levels without recognizing it. The good news is that carefully avoiding all low blood sugar episodes for several weeks can often restore awareness, resetting the body’s alarm threshold.
What Severe Hypoglycemia Does to the Body
When blood sugar drops very low, the brain loses access to its primary fuel. Neurons can’t function properly, leading to confusion, seizures, and in extreme cases, lasting brain damage. At the same time, the body floods the bloodstream with stress hormones, primarily adrenaline. Adrenaline levels can spike to more than six times their normal baseline during a severe episode.
This hormonal surge puts significant stress on the heart. Severe hypoglycemia can disrupt the heart’s electrical rhythm, causing abnormal beats, a racing heart rate, and potentially dangerous arrhythmias. The electrical recovery time of the heart (measured on an ECG) can increase by roughly 40% during a severe episode, which raises the risk of life-threatening rhythm disturbances. This cardiac risk is one reason severe hypoglycemia is treated as a medical emergency.
How to Treat a Low Blood Sugar Episode
The standard approach is the 15-15 rule: eat 15 grams of fast-acting carbohydrates, wait 15 minutes, then check your blood sugar again. If it’s still below 70 mg/dL, repeat the process. Good sources of 15 grams of fast-acting carbs include four glucose tablets, four ounces of juice or regular soda, or a tablespoon of honey or sugar.
The key word is “fast-acting.” Foods with fat or protein (like a candy bar or peanut butter) slow digestion and won’t raise your blood sugar quickly enough. Once your levels recover, eating a small snack or meal with protein and complex carbohydrates helps stabilize things and prevents another drop.
For someone who has lost consciousness or can’t swallow safely, oral food isn’t an option. Injectable glucagon, a hormone that triggers the liver to release stored glucose, is the emergency treatment. If you take insulin or are at risk for severe episodes, keeping a glucagon kit accessible and making sure the people around you know how to use it is essential.
How Continuous Glucose Monitors Help
Continuous glucose monitors (CGMs) check blood sugar every few minutes and can alert you before levels drop too far. Most devices offer three types of alerts: threshold alerts that notify you when glucose crosses a set number, predictive alerts that warn you before a low is expected to happen based on current trends, and rate-of-change alerts that activate when glucose is falling rapidly.
For most people, setting the low glucose alert between 70 and 75 mg/dL works well. If you’re at higher risk for severe lows (due to hypoglycemia unawareness, for example), a more conservative threshold between 80 and 100 mg/dL gives more lead time. During and after exercise, when the risk of dropping is elevated, some devices allow temporary thresholds as high as 125 to 150 mg/dL. Predictive alerts are especially valuable because they give you a window to eat something before symptoms even start.