What causes dry mouth? I'm 56yrs f/m, it happens really bad when I'm sleeping even with my mouth closed

Xerostomia. Some causes are; medications such as cold remedies containing antihistamines, diabetes, dehydration, stress/anxiety, anemia, salivary gland diseases, & tobacco use. Some home remedies are water, sucking on sugar-free candy, chewing sugar-free gum, use Fluoride toothpaste & mouth wash.
Many causes. There can be many causes for drymouth from taking medication to poor dental care. While sleeping your mouth can become dehydrated. Consult your doctor for possible causes.