Lying down removes the one advantage the body has against reflux during the day: gravity. Upright, anything that moves past the LES has a direct path back down, often without you noticing. Horizontal, that same content can sit right at the junction between stomach and esophagus, with nothing but the LES's own strength keeping it down — and for people with reflux, that strength often isn't enough. This is compounded by two other factors that shift when lying down: saliva production drops during rest, reducing one of the body's main ways of neutralizing acid, and if you've eaten recently, the stomach is also more likely to still be full, adding pressure against the valve. This is why timing (finishing meals well before lying down) and position (staying elevated and on your left side) matter more for reflux than almost any other single factor.
