When you have GERD, lying down can be an issue because it can allow your stomach contents to flow back into your esophagus. It is why people commonly experience symptoms of discomfort when sleeping or lying down (especially after eating a heavy meal 2-3 hours before bed). Keeping an upright position allows contents to stay within your stomach. It does not mean that you have to sleep upright to help relieve symptoms. Sleeping on your left side allows gravity to keep your stomach below your esophagus (decreasing the risk of acid reflux). Even if you experience symptoms, they may be more gaseous (which can be less distressing for you).
You could also sleep at an incline (or a combination of sleeping left at an incline). Sleeping at an incline (about 6-8 inches above) limits how often your stomach acid flows back into your esophagus, relieves heartburn, and allows the acid to flow back to your stomach quicker. However, you might want to avoid sleeping on your back because this can increase the amount of acid reflux that occurs during the night. Sleeping straight on your back allows stomach acid to flow easily into your esophagus and remain there. You may also want to avoid sleeping on your right side. Laying on your right side places your stomach above your esophagus. This position allows stomach acid to flow back into your esophagus. Studies also show that reflux symptoms tend to be more liquid-based when sleeping on your right side. Therefore, you may experience an increase in regurgitation, coughing, and choking.