The "official" ruling is "why do you want to have a rule about sleeping in armor? So you can ambush naked PCs and stab them when there ACs are really low? Man, my table hates that, but if it's fun for your guys then go ahead."
[sblock]As a minor personal note, I have slept in modern combat armor. It's a lot warmer than many people think. It's also very comfortable, once you shift yourself into certain positions.
While plate armor is made of metal plates, and is therefore less insulating, and a bit less comfortable, than modern kevlar, it has a nifty layer of padding to keep it relatively comfortable. It's also very well articulated to allow for a lot of flexibility in combat. This means that you can get yourself into surprisingly comfortable positions while wearing it.
Obviously, taking it off is a lot more comfortable than not. However, getting stabbed in the neck is a lot less comfortable than sleeping in armor. Which is why many soldiers in many real life wars have learned to sleep in their armor until they are in a very secure location well behind the front lines.[/sblock]