When blood sugar levels decrease below about 70 mg/dL, the body launches a cascade of hormonal responses to push glucose back up. This threshold marks the clinical definition of low blood sugar, or hypoglycemia. Below 54 mg/dL is considered severe. Understanding what triggers these drops, how your body responds, and what symptoms to watch for can help you act quickly and avoid complications.
What Your Body Does When Blood Sugar Falls
Your body doesn’t passively wait for you to eat something. As soon as blood sugar dips toward the 65 to 70 mg/dL range, three things happen almost simultaneously. First, your pancreas dials back insulin production so less sugar gets pulled out of your bloodstream. Second, it ramps up glucagon, a hormone that tells your liver to break down its stored sugar (glycogen) and release it into the blood. Third, your adrenal glands release epinephrine (adrenaline), which does double duty: it triggers the liver to release even more glucose and mobilizes backup fuel sources from muscle and fat tissue.
This counterregulatory system is remarkably precise in healthy people. The liver stores enough glycogen to maintain blood sugar for several hours between meals. Glucagon handles the quick release of that stored sugar, while epinephrine recruits raw materials like lactate, alanine, and glycerol so the liver and kidneys can manufacture fresh glucose through a process called gluconeogenesis. Cortisol and growth hormone also rise during prolonged low blood sugar, providing a slower but sustained boost to glucose production.
How Low Blood Sugar Feels
Symptoms fall into two distinct categories, and recognizing both matters because they signal different levels of severity.
The first group comes from your nervous system’s alarm response, the same adrenaline surge described above. These symptoms tend to appear earlier and are easier to notice: shakiness, trembling, a pounding heart, sweating, anxiety, tingling sensations, and hunger. They’re your body’s early warning system, essentially the physical sensation of adrenaline flooding your bloodstream.
The second group is more subtle and more dangerous. These symptoms come directly from your brain not getting enough fuel. They include difficulty thinking, confusion, warmth, weakness, drowsiness, and fatigue. Because the brain runs almost entirely on glucose, prolonged deprivation can progress to loss of coordination, seizures, loss of consciousness, and in rare extreme cases, death. The tricky part is that these brain-related symptoms are harder to self-detect, which is why the earlier adrenaline-driven warning signs are so important to recognize and act on.
Common Causes and Triggers
In People With Diabetes
For people managing diabetes with insulin or certain oral medications, low blood sugar is the most common side effect of treatment. Taking too much medication relative to food intake, skipping or delaying meals, or exercising more than usual can all tip the balance. Alcohol adds another layer of risk because it blocks the liver’s ability to produce new glucose and break down glycogen, essentially disabling your body’s primary backup system.
In People Without Diabetes
Low blood sugar can also happen in people who don’t have diabetes. Reactive hypoglycemia is the most common form: blood sugar drops two to five hours after eating, typically after a meal heavy in refined carbohydrates. The mechanism varies, but it often involves an exaggerated insulin response to the meal, overshooting what’s needed and pulling blood sugar below normal. People who have had gastric surgery are particularly prone to this because food moves through the stomach faster, triggering a rapid and oversized insulin release.
Other triggers in non-diabetic individuals include heavy alcohol consumption (especially on an empty stomach), liver or kidney disease, adrenal or pituitary gland disorders, severe infections, and certain medications including some antibiotics, lithium, and beta-blockers. Prolonged fasting and anorexia nervosa can also deplete glycogen stores enough to cause symptomatic drops.
Exercise
Physical activity pulls sugar from your bloodstream and from reserves stored in your muscles and liver. As your body rebuilds those stores after a workout, it continues drawing glucose from your blood, which is why blood sugar can drop four to eight hours after exercise. This delayed effect catches many people off guard, particularly if they exercise in the afternoon or evening and experience a low during sleep.
Nighttime Drops Are Especially Risky
Nocturnal hypoglycemia is common in people with type 1 diabetes and usually goes unnoticed because it happens during sleep. Nearly half of all severe hypoglycemic episodes occur at night. Signs you may have experienced a nighttime low include waking with damp sheets from sweating, feeling unusually groggy or irritable in the morning, or having a headache you can’t explain.
Repeated nighttime lows are particularly concerning because they can gradually blunt your body’s ability to detect future episodes, creating a dangerous cycle. They’ve also been implicated in cardiac rhythm disturbances during sleep. Checking blood sugar at bedtime, eating an appropriate snack before sleep, and working with your care team on insulin timing are practical strategies to reduce the risk.
The 15-15 Rule for Quick Recovery
When you feel symptoms of low blood sugar or confirm a reading below 70 mg/dL, the standard approach is straightforward. Eat or drink 15 grams of fast-acting carbohydrates. That’s about four glucose tablets, half a cup of juice, or a tablespoon of honey. Wait 15 minutes, then recheck. If you’re still below 70 mg/dL, repeat. Keep going until your blood sugar returns to your target range.
The key word is “fast-acting.” Foods with fat or protein slow digestion and delay the glucose spike you need. A candy bar is less effective than juice or glucose tabs because the fat content slows absorption. Once your blood sugar stabilizes, follow up with a more substantial snack or meal to keep it steady.
When Your Body Stops Warning You
Hypoglycemia unawareness is a condition where the usual early warning symptoms, the shaking, sweating, and racing heart, become muted or disappear entirely. This happens most often in people with long-standing diabetes, those on intensive insulin therapy, and anyone who has experienced repeated episodes of severe hypoglycemia. The body essentially recalibrates its alarm threshold downward, so you don’t feel symptoms until blood sugar is already dangerously low.
Pregnancy increases the risk significantly. Women with diabetes experience severe hypoglycemia and unawareness up to five times more frequently, especially during the first trimester. Other risk factors include kidney failure, heavy alcohol use, and frequent exercise without adequate fuel.
The condition is reversible in many cases. Carefully avoiding any hypoglycemic episodes for a period of weeks can help reset the body’s warning system, though this requires close monitoring and often adjustments to medication.
What Repeated Lows Do Over Time
The long-term picture is more nuanced than many people expect. Research suggests that moderate, repeated episodes of low blood sugar don’t appear to cause lasting cognitive damage when blood sugar is otherwise well-managed. In fact, the brain seems to adapt by becoming more efficient at using available fuel under normal conditions.
The real danger is compounding risk. Each episode of recurrent hypoglycemia dulls the counterregulatory hormone response, making the next episode harder to detect and harder for the body to correct on its own. Memory tasks that depend on the hippocampus, the brain region most sensitive to fluctuations in fuel supply, may perform fine day to day but suffer noticeably during a subsequent low. Anxiety levels also appear to increase specifically during the combination of a history of repeated lows and an active hypoglycemic episode, even when neither factor alone raises anxiety.
The practical takeaway: preventing lows matters not just for how you feel in the moment, but for preserving your body’s ability to protect itself during the next one.