Zaruthustran
The tingling means it’s working!
Note that I'm also not saying it's impossible to fall asleep in armour; just that you presumably don't get a good night's rest (again, I stress that I'm not talking about modern armour, but historical chain or plate). If there was no significant downside to keeping the armour on, historical warriors would have slept in their armour; those guys were not stupid.
I hear you. I think we're talking about different scale, though.
At army-level scale (in terms of both size/number of individuals, and timeframe/weeks on campaign trail) then I totally agree with you: sleeping in plate armor isn't feasible.
However, on adventuring-party scale--2 or 3 people in heavy armor, camping for a single night, with maybe a stretch of 2-3 nights in a row--I maintain that sleeping in armor is feasible. Uncomfortable, yes. But feasible.
Particularly with chain. Several of us in this thread have slept in chainmail. While I prefer flannel, I certainly got a decent rest out of it. Plate is less comfortable.
It's reasonable to disagree though. Really what you're advocating is giving a mechanical penalty to the characters who choose heavy armor. What I'm advocating is for that penalty to kick in only after successive nights, and for it not to be automatic.
In detail, what I propose is:
Sleeping in armor: make a con check after a certain number of nights of sleeping in armor. Add your proficiency bonus if proficient in the armor. Success = benefit from long rest. Fail = benefit from long rest but suffer 2 levels of exhaustion (a net cumulative 1 level of exhaustion for each failure). Note that being exhausted gives disadvantage on ability checks. The rules for exhaustion say you die after accumulating 6 levels of exhaustion, but I'd say you can't die as a result of failing this check.

Light armor: Easy Con check for 4th night sleeping in armor. Hard Con check for 5th & successive nights.
Medium armor: Easy Con check for 2nd night sleeping in armor. Hard Con check for 3rd & successive nights.
Heavy armor: Easy Con check for 1st night sleeping in armor. Hard con check for 2nd & successive nights.
Special: Miserable conditions (wet, dirty, etc.) or rigid armor (breastplate, half plate, splint, plate) imposes disadvantage to this check.