Chapter: What Happens Inside Your Body When You Lie Down?
Sleep position is a common subject of health advice, but online illustrations can make the body's internal processes look much simpler than they really are. The image accompanying this article suggests that lying on the right side allows stomach acid to move toward the esophagus, while lying on the left side keeps it inside the stomach. This captures an important idea about acid reflux, but it should not be interpreted as a universal rule that right-side sleeping harms the body.
To understand the difference, it helps to know how the digestive system works. When a person swallows food or liquid, it travels through the esophagus, a muscular tube connecting the mouth to the stomach. At the lower end of the esophagus is a muscular valve-like region called the lower esophageal sphincter. It normally relaxes to let swallowed material enter the stomach and then helps prevent stomach contents from flowing backward.
The stomach produces acid and digestive enzymes that help break down food. Its lining is adapted to withstand this acidic environment. The lining of the esophagus is different and is more vulnerable to irritation from repeated exposure to stomach contents. When stomach contents flow backward into the esophagus, the process is known as gastroesophageal reflux. When reflux occurs repeatedly and causes troublesome symptoms or complications, it may be diagnosed as gastroesophageal reflux disease, or GERD.
Reflux can cause a burning feeling behind the breastbone, commonly called heartburn. Some people experience a sour or bitter taste in the mouth, regurgitation of food or liquid, or discomfort that worsens after eating. Symptoms may be more noticeable when a person lies down, especially after a large meal, because gravity no longer helps keep stomach contents in the lower part of the digestive tract.
However, stomach acid does not simply pour into the esophagus every time someone turns onto the right side. Reflux depends on several factors, including the function of the lower esophageal sphincter, the position of the stomach and esophagus, the amount and timing of food consumed, and individual differences in anatomy and digestion.
Body position can influence the likelihood and duration of reflux. When a person lies on the right side, the relationship between the stomach and the junction with the esophagus may make reflux more likely or allow acid exposure to last longer. When a person lies on the left side, the stomach's position relative to the esophagus may make backward flow less likely. Clinical guidance and research reviews recognize left-side sleeping as a useful lifestyle measure for people who experience nighttime reflux.
This does not mean that the right side is harmful to everyone. People without reflux symptoms may sleep comfortably in different positions. Sleep comfort, joint pain, breathing issues, pregnancy, and other medical circumstances may influence which position is most suitable. Someone who sleeps on the right side without symptoms does not need to assume that their stomach is being damaged.
It is also important to distinguish reflux from normal digestion. Digestion continues while a person sleeps, and the body has several mechanisms to limit the movement of stomach contents into the esophagus. The illustration is a simplified explanation of one mechanism, not a real-time picture of what happens every night.
The useful takeaway is straightforward: if heartburn or regurgitation regularly occurs at night, changing sleep position may help. For many people with nighttime reflux, starting sleep on the left side is a reasonable step to try. But an individual's symptoms and medical history matter more than a dramatic headline.
Chapter: Why Sleeping on the Left Side May Reduce Nighttime Heartburn
The advice to try sleeping on the left side is not merely an internet trend. It is supported by research investigating body position and acid exposure in people with reflux. The evidence is not perfect, and it does not guarantee relief for every individual, but it provides a reasonable basis for this recommendation.
A systematic review and meta-analysis published in 2023 examined studies comparing left-side sleeping with right-side and back-sleeping positions among people with GERD. The researchers found that left-side sleeping was associated with reduced esophageal acid exposure and faster acid clearance compared with right-side sleeping and lying on the back. A randomized study of positional therapy also found improvements in nighttime reflux symptoms when participants spent more time sleeping on their left sides.
The anatomical explanation is relatively simple. The stomach is not a straight container, and the connection between the stomach and esophagus sits at an angle. When a person lies on the left side, the stomach's main reservoir of contents may sit below the esophageal junction in a way that reduces the opportunity for acid to move upward. Lying on the right side can place the junction in a less favorable relationship to the stomach contents. As a result, reflux may occur more readily or acid may remain in the esophagus for longer.