Translation:My brother sleeps in the bedroom all day.
I think because there's no adjective here in this sentence so that's why it's " hele dagen "
I like to think of "hele dagen" in its literal translation "all the day" because I find that thinking about the structure of Danish as being very much like that of older forms of English very helpful when I'm doing these translations.
Why is "sleeping room" incorrect? I am not a native English speaker but I thought we could say that :/
It's not used in English. We understand what you mean, but no one ever calls it a sleeping room. I've only ever heard "bedroom".
Think of it as 'the whole day'. 'He sleeps whole day' wouldn't make sense