If you flip it around (under chin and over head) and place the wide parts facing out, they keep your straps from marking your cheeks, too. Stretchy enough to keep your mouth closed, but has enough give so you can open your mouth if you need to. It's wide enough that it stays in place, even if you move around in bed. The one I found (in my icefishing gear!) is a little snug and a bit too warm for my liking, but with a little finagling, I think it's a cheap and effective alternative to the chin strap I have with clips that dig into the side of my face.
For those who live in hot climates and have no idea what I'm talking about:
