Oh yes, you can sleep without a bed, the ambush thing is really just an annoying artifical difficulty that can easily fixed by... fighting the thing that attacked you.
Well so you say be ambushed during resting in dungeon full of danger is "artifical difficulty" do you accept thats smart enemy will not wait until such ridiculously nonchalant character heal completely by simple resting to full power and you say player character will able fight back well in such dangerous situation, especially when we take a nap not in sewer full of rats but in an Dread Dungeon of Doom, quite interesting logic you have.
If ambushes for you are "artifical difficulty" then what is constant health regeneration from almost zero health to fully healthy in one keypress for all characters and without consequence and requirements?
I think adding health regeneration must have consequence for example you can regenerate health in such way only if current health not below certain percent
e.g you cannot simple wait one game hour to restore broken limbs and damaged internal organs (health is below of 50% of maximum) but contusions and bruises (Health is not below 80% of Maximum HP) you can.
Since health regeneration is not an skill but actual trait of character body biology better if regeneration will be tied to an Attribute or character Trait, doubtfully you can simple learn regeneration as card trick, but since attributes is out and all traits what we have is racial only, we can try bind health regeneration to certain skills as form of health care preparation during stay in camp (Firsts Aid, Medicine) for example Restoration school and Alchemy skill can affect how well you can care on your health during rest.