Acid reflux happens when stomach acid flows backward into the esophagus, the tube connecting your mouth to your stomach. Roughly 825 million people worldwide had gastroesophageal reflux disease (GERD) in 2021, making it one of the most common digestive conditions on the planet. The causes range from a weakened valve at the base of the esophagus to body weight, medications, and even how you sleep.
The Valve That Keeps Acid in Place
At the bottom of your esophagus sits a ring of muscle called the lower esophageal sphincter (LES). In healthy adults, this valve maintains a resting pressure of 10 to 30 mmHg, enough to keep stomach contents from creeping upward. When you swallow, the valve relaxes for about six to eight seconds to let food pass into your stomach, then tightens again.
The most common trigger for reflux episodes is something called a transient relaxation, where the valve opens on its own without a swallow. These spontaneous openings last more than 10 seconds, giving acid a wider window to escape. During these events, the diaphragm muscles that normally reinforce the valve also relax, and the esophagus shortens slightly, pulling the whole junction open. If your baseline valve pressure is abnormally low, or the pressure inside your stomach is unusually high, reflux becomes even more likely.
Once acid reaches the esophagus, your body relies on wave-like contractions (peristalsis) to push it back down. When those contractions are weak, acid sits in contact with the esophageal lining longer, causing more irritation and a stronger burning sensation.
Hiatal Hernias and the Anatomy of Reflux
A hiatal hernia occurs when the upper part of your stomach pushes through the opening in your diaphragm where the esophagus passes through. This displaces the junction between your esophagus and stomach, pulling it above the diaphragm. The muscles that would normally help squeeze the valve shut can no longer tighten effectively. The hernia also traps a pocket of acid at the top of the stomach that can’t drain back down easily.
Not everyone with a hiatal hernia develops reflux, and not everyone with reflux has a hiatal hernia. But when the two overlap, reflux tends to be more frequent and harder to control because the anatomy that normally protects against backflow is physically compromised.
How Body Weight Increases Stomach Pressure
Carrying extra weight around the midsection raises the pressure inside your abdomen, and that pressure pushes directly against your stomach. Research published in Gastroenterology found that waist circumference and the front-to-back diameter of the abdomen correlated strongly with intra-abdominal pressure, with a correlation coefficient of 0.68. Interestingly, BMI alone was a weaker predictor. This means where you carry weight matters more than your overall number on the scale.
When abdominal pressure climbs high enough, it can overwhelm even a normally functioning valve. This is one reason why reflux often worsens with weight gain concentrated in the belly and improves when that weight comes off.
Pregnancy and Reflux by Trimester
Pregnancy is one of the clearest examples of how hormones and physical pressure combine to cause reflux. Progesterone, which rises throughout pregnancy to support the uterus, also relaxes smooth muscle tissue, including the esophageal valve. Meanwhile, the growing uterus pushes the stomach upward and increases abdominal pressure.
A large meta-analysis found that GERD prevalence climbs steadily across pregnancy: about 26% in the first trimester, 33% in the second, and nearly 56% in the third. That sharp jump in the final months reflects the combined effect of peak hormone levels and maximum uterine size pressing against the stomach.
Medications That Weaken the Valve
Several common prescription drugs reduce the pressure your esophageal valve can generate. Calcium channel blockers (used for high blood pressure), nitrates (used for chest pain), and theophylline (used for asthma) all relax the smooth muscle of the LES. If you started experiencing reflux around the same time you began a new medication, the timing may not be coincidental. Your prescriber can often adjust the dose or switch to an alternative that’s less likely to affect the valve.
Smoking and Nicotine’s Direct Effect
Nicotine directly weakens the esophageal valve by acting on inhibitory nerve pathways that run through the vagus nerve. In controlled studies, nicotine caused a dose-dependent drop in valve pressure, with the highest doses reducing pressure by as much as 85%. This isn’t a subtle effect. Smoking also delays the stomach’s ability to empty and reduces saliva production, both of which make reflux episodes last longer and feel worse. People who quit smoking frequently notice a meaningful improvement in reflux symptoms within weeks.
Delayed Stomach Emptying
Gastroparesis, a condition where the stomach empties too slowly, creates a chain reaction that promotes reflux. When food sits in the stomach long after a meal, the stomach stretches and becomes distended. That distension makes it physically easier for acid to escape upward through the valve. You might feel uncomfortably full shortly after eating, bloated for hours, or nauseous, all signs that your stomach isn’t processing food on schedule. Diabetes is the most common cause of gastroparesis, but it can also develop after surgery or from nerve damage of unknown origin.
Why Reflux Gets Worse at Night
Lying down removes gravity from the equation. When you’re upright, gravity helps keep stomach contents where they belong. Flat on your back, acid can pool at the esophageal junction with nothing pulling it down. Nighttime reflux is also harder to clear because you swallow less frequently during sleep, and saliva production drops.
Sleep position makes a measurable difference. A study tracking esophageal acid exposure during sleep found that people sleeping on their left side had a median acid exposure time of 0.0% of the night, compared to 1.2% for right-side sleepers and 0.6% for those on their backs. Acid clearance time was also fastest on the left side: a median of 35 seconds versus 90 seconds on the right side and 76 seconds in the supine position. The anatomy explains this neatly. When you lie on your left side, the stomach hangs below the esophageal junction, so acid pools away from the valve rather than against it.
Food and Eating Patterns
Certain foods don’t cause reflux in the structural sense, but they can trigger or worsen episodes. High-fat meals slow stomach emptying and relax the valve. Acidic foods like tomatoes and citrus irritate an already inflamed esophagus. Chocolate, peppermint, coffee, and alcohol all reduce valve pressure to varying degrees.
Meal size and timing often matter more than specific foods. Large meals stretch the stomach, raising internal pressure. Eating within two to three hours of lying down gives acid a full stomach to work with and no gravity to keep it in place. Many people find that smaller, more frequent meals and an earlier dinner reduce nighttime symptoms significantly, sometimes without any other changes.
Stress and the Gut-Brain Connection
Stress doesn’t produce extra stomach acid, but it does amplify how you perceive reflux symptoms. The esophagus and brain communicate through a dense network of nerves, and psychological stress heightens the sensitivity of esophageal tissue to even small amounts of acid. People under chronic stress often report worse heartburn despite having the same objective acid exposure as when they’re relaxed. Anxiety also increases muscle tension and swallowing of air, both of which can raise stomach pressure and provoke more frequent valve relaxations.