Well maybe with her learning the demon language is making her both physically and mentally tired. After all it's not like they got to do a college course on Demon language and culture before they were summoned. (〜￣△￣)〜
I mean she is another world that and maybe her body tires easily from adjusting to the new surroundings such as climate, time and the whole world in general can both mentally and physically exhausting. (⊙…⊙ )
But way whether I am right or not you are right she does seem to have a LOT of chance sleep encounters with her demon. ε=ε=(ノ≧∇≦)ノ
10-12 hours is a normal amount, and from personal experience even if you sleep 15 hours you can still be tired ╥﹏╥
Is this just me who thinks that Rupia sleeps like waaayyy toooo damn much?
Like, okay, I get it, I sleep a lot too (I'm talking about 10-12 hours every day) but even I don't sleep that much!
Cause now every her and maou interaction is either he summons her or he stumbles upon her while she sleeps ┑(￣Д ￣)┍