Surgical Tape. I read Breath by James Nestor[1]. One of the suggestions is to put a one inch tape over your mouth during the night. It prevents mouth breathing and reduce instances of sleep apnea. I tried it; I start feeling refreshed after 3 days. You can give it a try, surgical tape is like $5USD.
1. Point 20 - https://www.chestergrant.com/summary-breath-by-james-nestor