Having us guess and get it wrong is one of Duo's teaching strategies. And you know, it feels so unfair to lose that heart, but it really does work.
Just think of it as a way to save lots of future hearts. :)
(And every now and again, I guess and get it right -- and I feel SO CLEVER. :)
It is just a German turn of phrase, it only sounds weird because you're thinking in English. To be fair, "Sleep in" makes no sense either. "Sleep in"? Sleep in what? It sounds like it should be followed by something, especially seeing as one should never end a sentence with a preposition, and yet that's what we say! Sleep in!
"sleep yourself off" makes a lot more sense than "Sleep in" does.
Indeed! I was taught -- sometime in the mid 1970's -- that "never end a sentence with a preposition" was an incorrect old rule that we might someday encounter... it was cooked up by people who were trying to make English conform to Latin rules, but it's plain wrong for a Germanic language.