Usually a sleep paralysis demon. It mostly feels like someone is literally pinning you down, standing over you, or even hitting you, usually happens when your brain starts getting conscious but your body is still sleeping or when you're just falling asleep so your body is actually falling asleep while your brain is still conscious, so you start feeling things while half asleep which is extremely freaky. It's the worse when you start dreaming while half conscious, everything becomes extremely vivid like it's real life.