There's an optional rule in Xanathar's Guide to Everything "When you finish a long rest during which you slept in medium or heavy armor, you regain only one quarter of your spent Hit Dice (minimum of one die). If you have any levels of exhaustion, the rest doesn’t reduce your exhaustion level."
Personally I ignore the rule because I have no clue how much more uncomfortable it would be to sleep in medium or heavy armor vs say studded leather. All I know is that when I've been backpacking sleeping on rock didn't stop me from getting a good night's sleep and other than a minute or so of stretching in the morning it was fine. I think a lot of penalties go back to outdated notions like having to have a crane lift you onto horseback if wearing plate.
