Morning breath occurs due to the production of hydrogen sulfide by bacteria that feast on dead cells from the tongue overnight. When sleeping, particularly through the mouth, saliva production decreases, preventing the usual rinsing away of debris. This process allows odor-causing bacteria to thrive, leading to the unpleasant morning breath many experience.

In her book "Gulp: Adventures on the Alimentary Canal," Mary Roach explains that while we sleep, the combination of reduced saliva and bacterial activity contributes to the funky smell of our breath in the morning. Regular oral hygiene practices are essential to mitigate this issue and maintain fresh breath throughout the day.