It has long been known that eating - and, especially, eating to large or fatty meal - shortly before lying down is a precipitant of GERD symptoms in susceptible individuals. In fact, even in people without noticeable GERD symptoms, eating a large meal before bedtime is sometimes associated with abdominal discomfort. At the least, their uncomfortable feeling of fullness is made worse when they lie down flat.
This situation is a simple matter of physics. When a person is standing or sitting, the esophagus is higher than the stomach, and the downward pull gravity helps prevent acid reflux. But when a person is lying down, the esophagus and stomach are essentially on the same level. In the position, acid can roll back up into the esophagus very easily, somewhat like the ocean coming in at high tide. When we lie flat, the contents of a full stomach tend to flow upward toward the LES, which is not a perfect barrier, even in people without GERD. For people with GERD, lyig down within three or four hours after eating may cause any remaining food and acid in the stomach to reflux into the esophagus - the stimulus for symptoms of GERD.
It's important to remain upright for at least three hours after a meal, and longer after a large or heavy meal. Another step to take to avoid reflux episodes is to elevate the head of the bed. For many people with GERD, nighttime is the time when they develop symptoms. This is true in part because when we are asleep, we produce less saliva and swallow less often. Swallowing saliva helps neutralize and clear away any refluxed acid; less swallowing means that the stomach acid is more concentrated - stronger - and therefore more likely to cause irritation. While elevating the head of the bed does not increase the amount of saliva or the frequently of swallowing. It does help reduce the flow of stomach acid into the LES. Elevating the head of the bed can often markedly improve GERD symptoms.