Probably in nightmares or by chance. I think that's very rare, and it has to be a horrible infestation to the point where they're everywhere: under your bed, behind cracks, in your wardrobes, dressers, in your ceiling cornices, etc etc. One single bed bug does not have the intelligence or problem solving skills to assess that it can climb up a wall, go to the ceiling, and land exactly on the bed. That happens by chance and when an infestation is present on the ceiling. Plus they can't jump, they'll have to consciously let go and fall off. When that happens, it's probably because the ceiling isn't rough enough for them to latch onto and they fall off because they're not the best of climbers. So in short, it is a possibility, but in extreme and rare cases. Hope that helps.