I haven't seen Rope Trick mentioned. Even at mid-levels that was our go-to camp spell in 3E, and from what I remember, little has changed. It's a ritual now, and I think the duration has been limited, but for a short rest it's likely all you'll need to keep safe for an hour.
No, D&D leather armour really is cuir bouilli style armour, although likely just a breast piece and pauldrons (with optional skirt of soft leather). The PHB even calls it out as hardened leather via boiling oil. The stuff was real, and in fact I might even include Macedonian hoplite linothorax as a material as well.
Neither chain nor plate are probably very comfy to sleep in, but I'd at least let a character wear the attendant padding and treat it as wearing padded armour if they insist on sleeping in something other than their skivvies.
Yeah, you seem to be right about Cuir Bouilli (I forgot about it earlier in the thread, and I didn't know the PHB specifically described leather armour in this way; well done PHB authors!). However, I can't find any information on how widespread it was. As I try to go for medieval-ness in my game-world, I'd be loathe to re-introduce linen armour from the ancient world, but of course in a fantasy setting it's at least a possibility.
I agree on wearing the padding to sleep in; I wouldn't allow PCs to sleep in Cuir Bouilli either; can't be any more comfortable than plate or chain. One or two people in this thread have said that soldiers can sleep in armour, but historically soldiers didn't generally sleep in chain or plate armour, even when on campaign, and if it wasn't a problem they would have done so. I'm not saying it's impossible, but you're not going to get the benefits of a full rest if you do so. This is all if you want to go with "realistic" historicity, of course.
Rigid leather isn't really rigid. It's less rigid than plate by far. And I've slept in rigid leather. It's no worse than sleeping in one's web gear (ALICE gear, or current acronymic equivalent)
Well, as I understand it, it's debatable how Cuir Bouilli was exactly made, so we're speculating as to its properties (people have made versions of it, but we don't know what is authentic). From my (limited) reading on the subject, it was most likely similar in rigidity to plate... but I don't think anyone knows, really. Do you any references for it being much softer than plate? I'd be interested to see them.
One or two people in this thread have said that soldiers can sleep in armour, but historically soldiers didn't generally sleep in chain or plate armour, even when on campaign, and if it wasn't a problem they would have done so. I'm not saying it's impossible, but you're not going to get the benefits of a full rest if you do so. This is all if you want to go with "realistic" historicity, of course.
Talk to some service members, and ask them if they've ever slept in their body armor.![]()
Of course it's not ideal. Plate, in particular, is uncomfortable when prone (depending on construction). But it's certainly possible to sleep in plate armor, for a short stretch of time (a night or two). The only reason to deny the benefit of a night's rest would be preference in game mechanics.
If a suit of heavy armor isn't readily available to personally test out (preferred), then take a gander at a similar fantasy world to D&D: Game of Thrones, as depicted on HBO. The Hound regularly slept in his (splint) armor.

(Dungeons & Dragons)
Rulebook featuring "high magic" options, including a host of new spells.