/soy/ - Soyjaks

Sometimes when I’m about to fall asleep and half asleep my thoughts turn into a kind of dream, then my half-sleep quickly turns into a nightmare, I feel very scary, I understand that I’m sleeping, but for a couple of seconds I can’t open my eyes and there’s a ringing in my ears, then I open my eyes, everything goes dark in my eyes. I'll lie there for a couple more seconds so it becomes normal. Why is that?