Even without "đã" you know it is on the past, right? That's how VN people talk, they use the tense particles only when are needed, if you can easily figure out easily the tense, like here, there's no need to use it.
And by "easily" means "easily for them", that can be different than "easily for you".
My wife is Vietnamese and most of the time I don't know the time on her sentences. She says "I have breakfast" and I never know if she just had breakfast, she's about to have breakfast or she will in some time.
Because "sleep" is not a countable noun in normal English, if the goal is to mimic the Vietnamese grammar, the term "nap" (which is countable) should be accepted: "I have a long nap," but this translation was not accepted. (English speakers would never use simple present tense for this sentence without some time element to indicate a habitual action, e.g. "I have a long nap after lunch.")