Sleep is 8 hours out of 24. This is one third of your life, so force yourself to nasal breathe during sleep = opening up your nasal airways for easier nasal breathing for your daily awake life

(from book “Breath: The New Science of a Lost Art by James Nestor”)

• Mouth breathing is a modern phenomenon for humans. 
• Mouth breathing is a relatively recent development, linked to factors like poor oral health, enlarged tonsils and adenoids, and changes in facial structure.