I don't know. Mushrooms make me want to stay as still as possible in whatever position I'm in, even if it's standing. Unless the person suffers from claustrophobia, I don't think it would be an unbearable experience.

It is possible to sort of sleep on psychs. I guess it's not quite sleep; it's more like a relaxed state of altered consciousness within the "normal" state of altered consciousness. I sometimes become alert again and realize I dozed off for a few minutes without losing track of time.