Morning breath is mostly a nighttime saliva problem.
Your mouth contains bacteria all the time.
During sleep, saliva flow drops.
That gives odor-producing compounds a better chance to build up.
Saliva normally helps clean the mouth
Saliva rinses food particles and bacteria away from surfaces.
It also helps control the chemistry of the mouth.
During the day, swallowing repeatedly moves saliva through the system.
At night, both salivary flow and swallowing decrease.
Bacteria keep working
Mouth bacteria break down proteins and other material.
Some of that activity produces volatile sulfur compounds.
Those compounds smell strong even in tiny amounts.
With less saliva washing them away, the odor becomes easier to notice by morning.
Mouth breathing makes it worse
If you sleep with your mouth open, airflow dries the mouth further.
That is why waking with a dry mouth and morning breath often travel together.
Why does brushing before bed help but not eliminate it?
Brushing and flossing reduce food particles, plaque, and bacterial buildup.
That gives the bacteria less material to work with.
But the mouth is not sterile.
Bacteria remain.
Saliva still drops overnight.
Some morning odor is therefore normal.
How is garlic breath different?
Garlic breath can come partly from compounds circulating in the bloodstream and being exhaled through the lungs.
Morning breath is mainly produced inside the mouth.
Two unpleasant smells.
Different plumbing.
What can help?
Good brushing and flossing, cleaning the tongue, staying hydrated, and treating chronic nasal congestion can all help.
Persistent severe bad breath can also come from dental or medical problems and may need professional evaluation.
Your mouth spent the night with reduced cleaning service while the bacteria remained fully staffed.
And that’s the short explanation.
Related explanations: Why do we wake up with dry mouth?, Why does garlic stay on your breath?, and Why do boogers form?