Why Do People Have Morning Breath?
During the day, we constantly swallow and eat/drink which washes away the bulk of the bacteria. We also brush our teeth and gargle our mouths which also helps.
Unfortunately, during the night, none of these activities occur which allow bacteria to proliferate and produce the smells associated with bad breath.
What makes this situation worse is that you can only smell other peoples' bad breath and not your own. Why is that? It's because our sense of smell becomes desensitized to smells that's around continuously.
In other words, if you smell someone's bad morning breath for a long enough period of time, you will no longer smell it!
Read more about morning breath in the Washington Post here.