Blood sugar drops when your body has more insulin than it needs relative to the glucose available in your bloodstream. This can happen for a wide range of reasons, from skipping a meal to taking certain medications to drinking alcohol on an empty stomach. Clinically, low blood sugar (hypoglycemia) is defined as a reading below 70 mg/dL, with readings below 54 mg/dL considered moderate and anything causing you to need help from another person classified as severe.
Skipping or Delaying Meals
The most common everyday cause of a blood sugar drop is simply not eating enough or waiting too long between meals. Your body continuously pulls glucose from your blood to fuel your brain, muscles, and organs. When you haven’t eaten in several hours, those glucose reserves start running low. If your last meal was high in refined carbohydrates, your body may have produced a surge of insulin that cleared glucose from your blood faster than expected, leaving you in a deficit before your next meal.
Reactive Drops After Eating
Paradoxically, blood sugar can also drop shortly after a meal. This is called reactive hypoglycemia, and it typically happens within four hours of eating. Your pancreas overestimates how much insulin it needs in response to a meal, especially one heavy in simple sugars or refined carbs. The excess insulin pulls too much glucose out of your blood, and you end up feeling shaky, lightheaded, or hungry again surprisingly soon after eating.
People who have had gastric bypass or other bariatric surgery are particularly prone to this. Food reaches the small intestine faster after these procedures, triggering a more rapid and exaggerated insulin response. Inherited metabolic conditions and, rarely, certain tumors can also cause post-meal sugar drops.
Diabetes Medications
For people with diabetes, medications are one of the most frequent triggers. Insulin itself is the most obvious culprit. If the dose is slightly too high for what you ate or how active you were, your blood sugar can fall quickly. The timing matters too: certain types of insulin peak hours after injection, which means a dose taken at dinner can cause a drop in the middle of the night.
A class of oral medications called sulfonylureas works by stimulating the pancreas to release more insulin into the bloodstream. Unlike your body’s natural insulin response, which adjusts based on how much glucose is present, these drugs push insulin release regardless. That mismatch is what makes them a common cause of unexpected lows, especially if you skip a meal or eat less than usual after taking your dose.
Alcohol and the Liver
Your liver acts as a glucose reserve, releasing stored sugar into your bloodstream between meals and overnight. Alcohol directly interferes with this process. When your liver is busy metabolizing alcohol, it becomes unable to produce new glucose through its normal chemical pathways. The breakdown of alcohol shifts the liver’s internal chemistry in a way that blocks the conversion of other fuel sources into glucose.
This is why drinking on an empty stomach or drinking heavily in the evening can cause dangerous blood sugar drops, sometimes hours later. The risk is especially high for people taking insulin or other diabetes medications, because the liver’s backup system for preventing lows is essentially offline while it processes the alcohol.
Exercise and Delayed Drops
Physical activity burns through glucose, which is why blood sugar often drops during a workout. But the less obvious risk is what happens afterward. Your muscles and liver need to replenish their glycogen (stored glucose) reserves after exercise, and they do this by pulling glucose from your bloodstream for hours. Studies in people with type 1 diabetes have documented post-exercise blood sugar drops occurring anywhere from 3 to 31 hours later, with most happening 6 to 15 hours after the activity ends.
This delayed effect catches people off guard. A morning run might cause a low in the middle of the night. An afternoon hike might trigger a drop the next morning. The more intense or prolonged the exercise, the larger the glycogen debt your body needs to repay.
Hormonal Deficiencies
Your body has a built-in defense system against low blood sugar. When glucose starts dropping, hormones like cortisol (from the adrenal glands) and growth hormone (from the pituitary gland) kick in to help raise it back up. If your body doesn’t produce enough of these hormones, that safety net fails.
People with adrenal insufficiency, where the adrenal glands don’t make enough cortisol, are prone to fasting blood sugar drops because cortisol plays a key role in telling the liver to release glucose. Research on people with pituitary disorders found that those deficient in both cortisol and growth hormone had significantly lower blood sugar levels than healthy individuals. In one study of over 100 patients hospitalized for severe hypoglycemia, roughly 20% had an impaired cortisol response and over 73% had an impaired growth hormone response.
Nighttime Blood Sugar Drops
Blood sugar drops during sleep are particularly concerning because you can’t feel or respond to the warning signs. Nocturnal hypoglycemia is more likely if you skip dinner, exercise in the evening, drink alcohol before bed, or are fighting an infection. Illness increases your body’s glucose demand while often decreasing your appetite, a combination that can quietly deplete blood sugar overnight.
People who have had a nighttime low once are more likely to have another. Signs that it happened include waking up with a headache, feeling unusually groggy, or finding damp sheets from sweating during the night.
Insulin-Producing Tumors
Rarely, a small tumor in the pancreas called an insulinoma can cause the body to produce insulin nonstop, regardless of how much glucose is in the blood. The hallmark pattern is a cluster of three things: symptoms of low blood sugar (sweating, palpitations, confusion, hunger), a confirmed low glucose reading at the time of symptoms, and relief once blood sugar is brought back up. About 80% of people with insulinomas experience drops primarily during fasting, though up to 20% also have episodes after meals.
Beyond the typical shaky, sweaty feelings, insulinomas can cause neurological symptoms like confusion, visual disturbances, and in extreme cases, seizures. These tumors are almost always benign, and the key diagnostic clue is that insulin levels remain inappropriately high even when blood sugar is dangerously low. In a healthy person, the pancreas would stop releasing insulin as glucose falls. In someone with an insulinoma, it doesn’t.
Other Medical Causes
Severe liver disease impairs blood sugar regulation because the liver is your primary glucose storage and production organ. When it’s damaged, it can’t keep up with the body’s demand for glucose between meals. Advanced kidney disease also increases the risk of blood sugar drops, partly because the kidneys play a role in clearing insulin from the body. When kidney function declines, insulin stays active in the bloodstream longer than it should, pulling glucose down further than intended.
Prolonged illness, sepsis, and severe infections can all cause blood sugar to plummet as the body burns through glucose at an accelerated rate. Eating disorders and extreme calorie restriction deplete glycogen stores, leaving the body without reserves to draw on between meals.