Why does my mouth stink when I wake up, even when I brushed at night? How do I fix this?

simple reason why:

When you sleep, your mouth dries out. When your mouth dries out, odor-producing bacteria proliferate. “When you sleep, your normal flow of saliva decreases,”.

Maintaining impeccable oral hygiene is both the best quick fix and long-term solution for a bad breath of any kind. Brush your teeth immediately before you go to bed at night, and don't eat or drink anything afterward. Doing so can introduce food particles that will be broken down over night.

Mouth is going to stink all the time. Anytime dig little amount of soft plaque from teeth and smell; it's certain to smell foul.

As for mornings, the different smell is due to increased alkalinity of saliva. One should drink water mixing that saliva in water in mouth every morning so as to reduce alkalinity in food pipe and stomach… so as to improve health.

To improve breath smell in general, make sure that you are not suffering from constipation.

Get yourself examined by a doctor with focus on your oral hygiene, respiratory system and the upper Gastrointestinal system.