Why Do I Get Heartburn at Night but Not During the Day?

Nighttime heartburn occurs because lying flat allows stomach acid to flow back into your esophagus more easily than when you're upright during the day. Gravity normally helps keep acid in your stomach, but this natural barrier disappears when you're horizontal. Additionally, saliva production decreases by up to 90% during sleep, reducing your body's ability to neutralize acid. Late evening meals compound the problem, as your stomach produces more acid to digest food just as you're preparing to lie down. The lower esophageal sphincter, which acts as a valve between your stomach and esophagus, may also relax more during sleep, making acid reflux more likely.