I'm playing hardcoe mode, and about halfway through Dead Money, my sleep gauge stopped increasing. It appears to be stuck at 0, though it doesn't display a number at all.
I remember that the Auto-Docs in Dead Money had an option to inject adrenaline, but I'm certain I never did that.
Now I'm out of the DLC and the gauge still doesn't do anything. The others appear to work as normal.

Nevermind, this was a super-simple fix. It started working again after sleeping.