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At night, your mouth becomes the perfect home for growing bacteria. Forgive us if we don’t talk to you until after you brush your teeth.
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Sources:
http://mentalfloss.com/article/51348/what-causes-morning-breath
http://www.ncbi.nlm.nih.gov/pmc/articles/PMC373626/
http://esciencecentral.org/journals/role-of-human-lactoferrin-in-oral-diseases-2332-0702.1000e108.php?aid=24826
http://www.ncbi.nlm.nih.gov/pubmed/15222464
https://books.google.com/books?id=cKWEGOzGz7MC&pg=PA22#v=onepage&q&f=false
http://www.ncbi.nlm.nih.gov/pubmed/12847442
http://www.scientificamerican.com/article/beat-bad-breath-keep-mouth-bacteria-happy/?page=2
You know the drill. You wake up in the morning, you stretch, you yawn, you realize that it kind of smells like something died in your mouth overnight. But actually, if it makes you feel any better, it's really the opposite. 

Your mouth makes that awful smell in the morning because whole colonies of bacteria have been growing in there all night. Bacteria really, really love the inside of the human body and your mouth, with its warmth and constant supply of food particles, is no exception. 

Your mouth bacteria aren't all bad because among other things, they help keep more dangerous outside bacteria from invading. But in many ways, they are the lesser of two evils because you can also thank your mouth bacteria for fun things like cavities and gum disease. 

Most of the time, your saliva helps keep the colonies under control by sweeping them and the food particles they eat down your esophagus. Your saliva also contains some anti-microbial agents like lactoferrin which works by stopping bacteria from absorbing iron properly.

But at night, you don't produce as much saliva. Your mouth becomes this warm, humid breeding ground with plenty of tiny food particles and dead cells for all the growing colonies to feast on. 

As they eat, the bacteria produce a bunch of different odor-causing compounds as waste products, including hydrogen sulfide which is the stuff that makes rotten eggs so stinky; methanethiol also known as fart smell; isovaleric acid which is what you're smelling on your feet after wearing boots all day and also happens to be found in beer, and cadaverine which is responsible for the scent of rotting flesh.

So when someone tells you that your breath smells like a corpse in the morning, that's not really an exaggeration. 

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