My sim with the vampiric slumber power does tend to wake up unexpectedly the first time I tell her to hibernate. The second time seems to stick though. I usually use hibernate to keep them in their coffins until sunset. All of the other members of that household tend to hibernate until I cancel it and I only have to tell them once, but the one with that power seems to always wake up once.
But if you play any other household, when you return to the vampire one they'll all be in random positions, not where you left them, including hibernate.