While you are sleeping is it possible to faint?
Do you mean because you’re lying down (usually) when you sleep (and faints are much less common in that position)? Or, do you mean how would you, or anyone else, know if you fainted during sleep? I assume the latter.
It’s like the tree falling down in the forest. I guarantee that circumstances can occur during sleep that would cause a faint in an awake individual (e.g. certain heart rhythm disturbances). But how y’gonna know if it happened? So, does that tree make a sound?