It gets a bit complicated because there are two different aspects of the discussion. One is the state of those in heaven before the resurrection and one is the state after. As Paul says, "Listen, I tell you a mystery: We will not all sleep, but we will all be changed— in an instant, in the twinkling of an eye, at the last trumpet. For the trumpet will sound, the dead will be raised imperishable, and we will be changed. For the perishable must be clothed with the imperishable, and the mortal with immortality."
Here Paul seems to refer to the people not on earth, who we typically consider to be dead, to be asleep. I don't claim to be an authority but he may mean those "sleeping" in heaven, as in not lucid, like in a dream that makes them happy, rather than truly conscious in the heavenly realms. Now after the resurrection the sleepers and the conscious ones will both be changed and the sleepers presumably be awake in the new world.