Let's face it: HP is an outmoded definition of measuring health in a modern society, where on a personal level, health never really gets damaged significantly or really lost. Sure there's hospitals and such, but combat-level injuries are rare among most people.
But there's another measurement that could result in death or glory among just about every single person, and whether you like it or not, it's sitting there inside you, waiting for you to assign it a value of some sorts. I put forth the concept of SP, or sanity.
There's a slow SP drain during the winter, as everyone's bundled up indoors and the outside is all melancholy. I end up taking an SP hit every now and then when I'm faced with a retarded customer on the phone, and I get a bit of it back when I talk to my dad.
Same thing most likely goes for you. SP changes all the time. It comes and it goes, and if you stay a 15-second night at an inn, it's most definitely not restored. And if SP hits 0, you're screwed just as hard, if not harder, than if your HP hit 0.
Have fun with that.