[achileas]

Not OP, but old dentist made and sold these. It is a custom-fit mouthpiece that forces you to keep your lower jaw forward while you sleep, preventing airway collapse. It’s the same thing you’re taught to do for CPR to force the airway open.

[robotguy]

Oh man, I had something like that as a kid, for my overbite. It was called a bionator and was like a retainer on steroids.