Mouth open sleeping? Dumb Q

[Kristincookie]

This is probably a dumb question but is it ok my guy sleeps with his mouth open sometimes(often)? He doesn't have it open during the day or even when he takes a nap when he basks. Just at night he seems to usually have his mouth open. No fatty lip or respiratory issues, so I assume ok. I just worry about everything.

 

[Acre]

Low humidity could be an issue. I'm in AZ and it even messes up my nasal passages.