In learning 5E for the group I'm running, occasionally I come across a rule that really blows me away for the effect it should have on the Setting (but doesn't). It's like the developers didn't care to think the implications all the way through in their goal to make things more comfortable for PCs.
I think the idea that every rule should be analyzed in terms of setting implications has largely fallen out of favor in modern RPG design, which is much more focused on game impact. The rules by which the game is played need not be treated like physical laws of the setting.
Today's mechanic of choice is healing. In 5E RAW, characters recover all their hit dice and all hit points every night.
Only half your hit dice (and all your hit points) by RAW, actually. Not that that changes anything about the point under discussion, I just thought I should point it out since you mentioned you’re learning 5e.
Think about the setting implications of this for a minute. No matter what you do, how badly you injure yourself, as long as you are not dead, you will be fully healed the next day as long as you get to bed for 8 hours. You could be starved and tortured for months, go to bed. You could fall off a building, go to bed. You could be impaled on a spike, go to bed. Everything is made better if you go to bed.
Not necessarily so. Hit Points only really measure your ability to keep fighting. The are not the only mechanic for representing a character’s wellbeing. For example, it takes multiple weeks of bed rest to recover from a disease without magic or end an effect that is preventing you from regaining hit points (the exact amount of time depends on which version of the Downtime rules you’re using, and the results of your Constitution saves.) Personally, I don’t think hit point loss would be the best way to represent the effects of long-term torture. I would recommend using exhaustion levels and/or lingering injuries from the DMG for the physical effects and Flaws or maybe madness levels for the psychological trauma, although I’m not really a fan of the madness rules myself. If you did want to keep it to hit point loss, I would recommend ruling that the extent of the trauma was so great, it prevents you from regaining hit points, which could be ended via rest during downtime.
Now I get that any DM can house-rule (and I'm curious who does) special situations. "I'm sorry, but your character suffered pretty extreme trauma this session; I'm going to say that you without magical healing you'll need a week to recover your hit dice." But it's not like this is even suggested.
I wouldn’t really call this house ruling. 5e is written with the expectation that the DM will make rulings like these to cover situations the rules don’t. Again, the idea of rules-as-physics-engine has pretty much gone the way of the dodo in modern RPG design. The 5e rules cover what is expected throughout the typical course of play, and provide a strong framework for the DM to make their own rulings when needed. I would argue that this is playing to the medium’s strengths. Computers do a better job of running rules systems that function as comprehensive physics engines. Human DMs do a better job of adjudicating on the fly based on simpler scaffolding.
I've heard that long rest taking a week is a house-rule losted in the DMG, but also that it's a sloppy solution because it doesn't mesh well with other things that require long rest (like spells).
I think there are places where the longer rest times (and the shorter ones!) in the DMG are appropriate to use, but I agree with you that they are not a good solution to the narrative problems that treating hit points as meat can cause.
Those of you who are used to older editions, what justification to you use for nightly healing? Or if not, do you have your own house-rule?
I started with 3e, so not sure if you’d consider that “older editions” or not, but I certainly would. The justification I use is that hit point loss doesn’t necessarily represent physical trauma. It is an abstract representation of one’s ability to stay in a fight, combining a certain degree of physical wellbeing with stamina, skill at avoiding or reducing harm from an attack, luck, and potentially even divine favor. It is also not the only, or even best, way to represent physical wellbeing. There’s also death saving throw successes and failures, exhaustion levels and other conditions, long term injuries, flaws. The rules have all sorts of ways to represent harm besides just HP loss, many of which require more than just a long rest to recover from.