That's perfectly normal, a safety mechanism to keep people from thrashing about and acting out their dreams. When it gets out of sync, people experience sleep paralysis. A person experiencing sleep paralysis can still see the actual room, but they also may have visions similar to Leslie. And that's exactly why these are so frightening, because the individual is virtually unable to ascertain at what point reality is leaving and dream imagery is coming. It was a motif repeated in other images of nightmares as well, and for good reasons.
Sleep experts know that the paralysis causes that pressure on the chest.
While people dream, none of their chest muscles, except the diaphragm, can move. So if they're partly awake, it feels as if they're not breathing normally, and something's pressing down on them. But even then they were way behind the local lore. Humans have been suffering from it forever, universally believing that something supernatural was upon them. You may have a demonic interpretation somewhere else but the experiences themselves don't seem to be different .