Smoking cigarettes may contribute to weakening of the esophageal sphincter and it would be easier to suffer an acid reflux disease. It is a tough muscle at the bottom of the esophagus which keep the food in the stomach and not allowing the contents of the stomach to make its way to the esophagus. After a meal, your stomach fills with acid so that the food digesting process can begin. If you choose to lie down, than the stomach acid is likely to enter into your esophagus. Gravity also will contribute to this positive effect, either by sitting or standing upright. The stomach acid will remain in the stomach where it is meant to be.