What Causes Morning Breath? Morning breath is a topic of many jokes, and a real boon to the mouth wash and toothpaste industries. But what exactly causes the foul taste and odor in your mouth when you wake up in the morning? The most basic answer to this question is gravity. If you’ve eaten fewer than three hours before you go to sleep, your stomach hasn’t had time to digest all of your food. When you lie down, gravity causes the gastric juices that are busy digesting your food to come up into your throat. Because your airway and food pipe are side-by-side, they share an adjoining wall. Those gastric juices that have backed up in your food pipe actually permeate that wall and enter into your air pipe. These juices can irritate your larynx, and cause your voice to be hoarse and your breath to turn foul. In addition, gastric juices are acidic enough to burn the mucous membranes in your throat, resulting in a sore throat that occurs in the mornings, but lessens throughout the day. Morning mouth tends to worsen with age, because as you get older, the top of your esophagus, the thin tube that leads from the mouth to the stomach, loosens. With the softening of the top of this tube, gastric juices can more easily escape from the esophagus into the air pipe. Another contributing factor to morning breath is infrequent swallowing during sleep. Swallowing allows us to keep our mouth relatively free from odor-producing bacteria. But when we’re asleep, these bacteria can thrive, contributing to the bad taste in our mouth when we wake up.