I think this is a terrible idea. For one thing, it doesn't even make sense. People can get a good night's rest when camping or in a cave. What do you think our ancestors did, before there were houses and beds to sleep in? But more importantly, it's a terrible idea from a game design perspective. First level characters can have single digit hp, making them extremely fragile. Now they might end up being stuck with half of that, until they can get back to town. So you can have wizards or rogues with as few as 3 HP after resting. Sure, they can get magical healing, but this is also a terrible idea for that reason. Since characters can't recover more than half of their hp from resting in a dungeon, the cleric (or other healer) is going to have to spend his healing resources first thing in the morning healing everyone from yesterday's injuries.