My question is what causes dry mouth while sleeping?

Mouth breather: Usually waking with a dry mouth is caused from breathing through the mouth or sleeping with your mouth open. However, some medications can cause dry mouth, if you don't think you are a mouth breather. Is there a medication you take at night before bed? If so, talk with your pharmacist about whether dry mouth is a side affect.

Multiple reasons: Salivation is reduced by about half wren we sleep. We tend to snore/ mouth breathe when we sleep, drying tissues. Some meds cause dry mouth. Some systemic diseases and syndromes cease dry mouth. See your dentist for exam.