When is your tongue like your socks? When it harbors bacteria that make bad smells.
Almost all of the odor of bad breath (called halitosis by doctors) comes from your tongue. Some may come from the gums if you have gum disease. Some comes from the rest of the mouth, or the throat, the nose, or the stomach, but by far the most fertile ground for breeding halitosis bacteria is the broad top surface of the tongue.
The top of the tongue has many little crevices that bacteria can live in. Most of the bacteria that cause bad breath are anaerobes, bacteria that grow best without air. As you sleep, they grow undisturbed, and you wake up with morning breath.
As the bacteria digest food particles and dead skin cells on the tongue, they create waste products that include the hydrogen sulfide that gives rotten eggs their odor, the methyl mercaptan that is present in skunk scent, and other sulfur containing small volatile molecules that result from the breakdown of sulfur containing proteins.
Amino acids that don’t contain sulfur also break down into smelly molecules. Tryptophan breaks down into indole. It is one of the molecules that give feces their odor.
Another breakdown product of tryptophan is skatole, which is just indole with an extra methyl group added. It is also found in feces, and contributes to that characteristic odor.
So please do us all a favor, and brush the top of your tongue in the morning when you brush your teeth.