Studies show that sleeping on your left side is best. The reason is not entirely clear. One hypothesis holds that right-side sleeping relaxes the lower esophageal sphincter, between the stomach and the esophagus. Another holds that left-side sleeping keeps the junction between stomach and esophagus above the level of gastric acid. In a study in The Journal of Clinical Gastroenterology, scientists recruited a group of healthy subjects and fed them high-fat meals on different days to induce heartburn. Immediately after the meals, the subjects spent four hours lying on one side or the other as devices measured their esophageal acidity. Ultimately, the researchers found that “the total amount of reflux time was significantly greater” when the subjects lay on their right side.