Every morning, take a dedicated toothbrush and gently brush the night guard with some toothpaste. It should never turn yellow or worse. Mold has never grown on my previous ones, but the first got pretty strange. There was no sensation of unpleasant taste, but an appliance hugging the gums overnight can’t be allowed to harbor bacteria.
If there is actual mold, you might have to get a new one. I’m not sure. Diluted bleach may or may not damage it. Vinegar might help. Check with your dentist. Night guards are too expensive to neglect.
My sleep is OK nowadays. That’s a blessing. I think it’s a combination of exhaustive exercise and a cleaner diet. No coffee at night and no red meat, for starters. Stress levels haven’t changed, but I gauge my health by heart palpitations.