P.U.! It happens to most of us from time to time: Morning breath!
The culprit behind bad breath in the morning is bacteria, tiny little critters too small for you to see that live naturally in and on all sorts of places -- including your mouth! The bacteria in your mouth get stinky at night because they have more of a chance to reproduce and multiply themselves, which lets off a yucky smell.
Even though you have bacteria in your mouth all day, you’re constantly eating, drinking, producing saliva, and swallowing, so the bacteria don’t have a chance to get out of control. At night, however, your body produces less saliva, you swallow less often, and you don’t have food or drink to wash them down. – This gives the natural bacteria in your mouth the opportunities they didn’t have all day, and sometimes leaves you with icky morning breath! (At least, until you brush your teeth!)