You mean that all this time I've been getting up after stamina/hp was full and I could have been getting the same regen buff from sleeping? I've never heard anyone mention this in discord so it may be common knowledge. I guess I'll have to try it out. But the point remains that the dev hates stamina regen. I made a suggestion in discord to change the regen from +1 to +1% which wouldn't really do much, but since you have 100 stamina when you start that is actually 1%. When you gain more stamina thru leveling and use more stamina with tougher monsters that +1 is no longer 1%. If you recall, the rested/well rested/hyped buffs used to be based on percentages and now they are flat numbers. This happened after I suggested the change. Not that it was in direct retaliation against me, but clearly the dev hates stamina regen.
I know well the war the Dev has waged against stamina. I still believe that the dev was/is trying to make the game more challenging, but they seem to have settled on crippling stamina as much as possible to 'create challenge'. It doesn't feel more challenging, just tedious.
The reason why stamina regen will never be a percentage (IMO) is pretty much what you said. A larger stamina pool means that 1% is no longer 1 and would negate a lot of the tweaks and debuffs to stamina regen.
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The first time I noticed the rested/well rested buff from sitting, was when the dev had first introduced the hot/scorching debuff, which added -1 or -2 stamina regen. When I was mining brimstone and forgot to remove my steel armor, I would spend stupid amounts of time sitting around and started noticing that I would have a rested or well rested buff randomly appearing. I never thought much about it and never thought to mention it on the forums here.
I don't know if it was a common knowledge thing or not, I stumbled into it on my own but just never thought about it much, mostly because I didn't want to sit around forever just to gain a short buff to stamina regen.