In this example, “as” is a preposition. However, it can also be used a conjunction.

Sleeping is is the same thing as wasting time. (Correct) (Preposition)

As you said, we should study instead of sleep. (Correct) (Conjunction)

We say “sleeping is the same as wasting time”. This is parallel form; we could say that it is short for the theoretical phrase: “sleeping is the same as wasting time is”. “As” is not considered a preposition, although in practice, it really doesn’t matter. “Like” is considered a preposition, and so governs pronouns in the object case, while “as” is supposed to take pronouns in the subject case: “He is as I (am)”, “He is like me”. In real life, the tendency is to use the object case as an emphatic pronoun (as [they do] in French grammar).