I always thought they slept because thier task was competed. They were bonded to the Masamune so that Lavos would be destroyed. Remember, the red knife's sole purpose was to destroy the Mammon Machine, which was connected to Lavos. When the Masamune was made the two still had thier purpose of destroying Lavos. When the group did so, Frog either laid the Masamune to rest, or kept it with him until his death, where it would have been either laid next to his grave or sealed away. This is when Masa and Mune would have slept, knowing thier task was completed they slept deeply, not noticing the corruption taking place.