My dentist, the brilliant Dr. Erik Graham, told me two years ago that the place for the tongue when breathing at rest is on the roof of the mouth.
It’s supposed to press up towards the heavens on the bone of the hard palate, NOT pressing forward on the upper teeth, mind you, unless you want to encounter some expensive dental work!
I had always been a mouth breather with the tongue laying on the jaw bone. Mouth breathing at rest and at night resulted in dry mouth and dehydrated, tight, and inflexible tissues inside the mouth, including the soft palate.
Making this simple change in tongue position has resulted in increased range, resonance, and beauty in my singing. It has also given me new flexibility in my facial expressions and better posture. Best of all, my body is much more calm now.
Of course, when we’re singing, it’s impossible to maintain this upward-pressing tongue position, because the mouth is open and the tongue is moving to form words. But between phrases it’s possible to practice this calming and re-hydrating breath.
Try it and see what happens!
Note: if you want more information on this subject from a wide variety of perspectives, just search “tongue position while breathing.”