In case it's (already?! :) been deleted by the time you read this, I'll just reproduce here the key point made in @Gustavson's comment...
ESL books almost always show the adverbs already and yet as exclusively associated with the present perfect, so it's no wonder that the OP found this use new.
Before reading that I couldn't really see why OP thought it was somehow "unusual" to use already with "non-perfect" verb forms. But thinking about it now I realise this is because already (and its "counterpart" yet) usually imply some kind of juxtaposition, contrast between two different times.
OP's example probably doesn't reflect actual "amazement" that they should sleeping so early - but in general, X has already happened implies that X wasn't expected to happen yet (not until later). Just as X has not yet happened tends to imply it was expected to have happened earlier.
The implication of the above is that whenever we use already or yet, it's quite likely the context will encourage us to use a "perfect" verb form for one of the two different times being juxtaposed/contrasted. That's simply because as Wikipedia points out...
The perfect tense or aspect (abbreviated perf or prf) is a verb form that indicates that an action or circumstance occurred earlier than the time under consideration, often focusing attention on the resulting state rather than on the occurrence itself. (Italics mine.)
So although in semantic terms, the primary reason for using perfect forms is usually focusing attention on the resulting state, syntactically it often turns out that we want to refer to something that happened before it was expected using a perfect form. But this is just a loose tendency caused by the interaction of semantics and syntax, not a grammatical principle as such.
I'll also just make the point here that because native speakers tend to prefer simpler tenses where they have a choice, you're as likely to hear I already did it where learners might think I have already done it is somehow "more correct". As ever, when it comes to perfect verb forms, the best advice for learners is to look for ways to avoid them, rather than ways to use them more often.