Causes of Heartburn At Night. In addition to normal heartburn causes, such as eating irritating foods and overeating in general, there are a few specific causes of heartburn at night which may increase your symptoms: Body Position: Body position is the number one to most cases of heartburn at night. The reason is simple: when you are lying flat, fluid no longer has to go against gravity to leave the stomach and enter the esophagus (heartburn is caused by the stomach’s contents moving into the esophagus).