A blood glucose level below 70 mg/dL (3.9 mmol/L) is considered low. This is the threshold used by the American Diabetes Association and most medical guidelines, and it applies whether or not you have diabetes. Below this number, your body starts sending warning signals that it needs fuel, and the lower it drops, the more serious those signals become.
The Three Levels of Low Blood Sugar
Not all lows are equally dangerous. The ADA classifies hypoglycemia into three levels based on how far glucose drops and how it affects your ability to function.
- Level 1: Glucose between 54 and 69 mg/dL (3.0 to 3.8 mmol/L). This is a mild low. You’ll likely feel it, but you can treat it yourself with food or a sugary drink.
- Level 2: Glucose below 54 mg/dL (3.0 mmol/L). At this point, your brain isn’t getting enough fuel to work properly. You need to act immediately.
- Level 3: A severe episode where you’re too confused, disoriented, or unconscious to help yourself. This is defined by needing another person to assist you, regardless of the specific number on a meter.
The jump from Level 1 to Level 2 matters more than the numbers suggest. Above 54 mg/dL, your body is mostly reacting with adrenaline-type symptoms: shaking, sweating, a racing heart. Below 54, the symptoms shift to your brain: confusion, slurred speech, difficulty concentrating, and behavior that can look like intoxication.
What Low Blood Sugar Feels Like
Your body responds to falling glucose in two waves. The first wave is driven by stress hormones. You might notice sweating, shakiness, a pounding heartbeat, sudden anxiety, or intense hunger. These are your body’s alarm bells, designed to push you toward eating something.
If glucose keeps dropping, the second wave hits. This is your brain running short on its primary fuel. Symptoms include weakness, dizziness, blurred vision, trouble concentrating, and confusion. In extreme cases, seizures or loss of consciousness can occur. Some people describe feeling “off” or unusually irritable without realizing their sugar has dropped.
Older adults often experience fewer of the early warning symptoms and may not notice a low until it’s already affecting their thinking. This makes regular monitoring especially important with age.
Normal Range vs. Low Range
A normal fasting blood glucose for someone without diabetes falls between 70 and 99 mg/dL (3.9 to 5.5 mmol/L). That said, some healthy people without diabetes can dip to 50 or 60 mg/dL without feeling symptoms, particularly overnight or after prolonged fasting. Cleveland Clinic notes that values between 50 and 70 mg/dL can be normal for people without diabetes in certain contexts.
The distinction matters because a reading of 65 mg/dL means something different for a person on insulin than for someone who simply hasn’t eaten in a while. For people with diabetes, any reading below 70 mg/dL warrants attention and a snack. For people without diabetes, doctors typically look for three things before diagnosing a true hypoglycemic disorder: symptoms consistent with low blood sugar, a lab-confirmed low glucose reading at the time of those symptoms, and resolution of symptoms once glucose is brought back up. This set of criteria is known as Whipple’s triad.
Lows After Eating
Reactive hypoglycemia is a drop in blood sugar that happens within four hours after a meal, most commonly two to three hours after eating. It occurs when your body releases too much insulin in response to a meal, overshooting the amount needed. The result is a sugar crash that can cause shakiness, lightheadedness, and brain fog even though you recently ate.
These episodes rarely push glucose low enough to cause severe symptoms like loss of consciousness. If you’re regularly feeling shaky or foggy a couple of hours after meals, it’s worth checking your glucose during those moments to see if your levels are actually dropping or if something else is going on.
Lows During Sleep
Nocturnal hypoglycemia is a drop below 70 mg/dL that happens while you’re asleep, and it can be hard to catch because you’re not awake to notice the usual warning signs. Clues that it’s happening include waking up drenched in sweat, restless or irritable sleep, nightmares, or a racing heartbeat during the night. You might also wake up with a headache or feel unusually tired the next morning.
A partner may notice changes in your breathing pattern, trembling, or clammy skin while you sleep. If these signs show up repeatedly, nighttime glucose monitoring (or a continuous glucose monitor) can help identify what’s happening.
How to Treat a Low
The standard approach for a mild to moderate low is the 15-15 rule: eat 15 grams of fast-acting carbohydrates, then wait 15 minutes. If you still feel symptomatic or your glucose hasn’t come back up, eat another 15 grams. Examples of 15 grams of fast-acting carbs include four glucose tablets, half a cup of juice or regular soda, or a tablespoon of honey.
The key word is “fast-acting.” A candy bar or a peanut butter sandwich won’t work as quickly because fat and protein slow digestion. You want pure sugar that hits your bloodstream fast. Once your glucose stabilizes, follow up with a small meal or snack that includes protein or complex carbs to keep it from crashing again.
For Level 3 episodes where someone is unconscious or unable to swallow safely, do not put food or liquid in their mouth. This is when emergency glucagon, an injectable or nasal medication that rapidly raises blood sugar, is used. If you take insulin and have a history of severe lows, keeping a glucagon kit accessible (and making sure people around you know how to use it) is a practical safeguard.
Common Causes
In people with diabetes, the most frequent trigger is too much insulin or other glucose-lowering medication relative to what you’ve eaten or how active you’ve been. Skipping a meal, exercising harder than usual, or drinking alcohol on an empty stomach can all tip the balance.
In people without diabetes, low blood sugar is less common but can result from prolonged fasting, excessive alcohol intake, certain medications, or (rarely) conditions like an insulin-producing tumor or adrenal insufficiency. Reactive hypoglycemia after meals is the most common pattern in otherwise healthy people, and while uncomfortable, it’s generally manageable through dietary changes like eating smaller, more frequent meals with balanced macronutrients.