I’m not that well versed on drifter stuff as I was unsubbed for a long period when they were the dominant threat, so someone else could probably explain it a lot better than I could. From what I understand though, sleepers were always present in wormhole space, as they were an offshoot of the Jove that explored it. They were “asleep” more or less hence the name, but our presence there woke them up and now they’re the rats you fight in W-space. Drifters I believe were the result of CONCORD prodding around in space somewhere and happening upon them, although I could be wrong because I believe they and the trigs were discovered through similar means. I’m not sure what their origins are, I just know that they’re extremely dangerous and shoot just about everything.
If you wanted a more accurate reply, I’m sure there’s someone that browses the Eve Fiction boards that could tell you much more than I could.