Some medications can cause your mouth to become dry overnight.
6. Allergy Season
Allergies, can lead to bad breath, too. As your postnasal drip drips down the back of your throat it becomes a food source for bacteria.
All the food you eat begins to break down in your mouth. As foods are digested and absorbed into the bloodstream, they are eventually carried to your lungs. Thus gives you bad breath. Onions anyone?