One thing that bothers me is my inability to sleep on my back if I put the pillow pressure directly on the back of my head, where my head connects to my neck, right under where the skull softens up back there. I've also though maybe it's that the blood is pooling in that area, but it gives some gnarly nightmares if I fall asleep that way. Started out of nowhere in my mid-20s, and still affects just about every time I accidentally fall asleep that way a decade later. I became a side sleeper or a back sleeper where my head is turned at least 45 degrees to a side.
I had two dreams where I literally woke up and got to work immediately started coaching myself to forget whatever I just dreamt and not think about it too much, because I know what causes it (the sleeping position), and that position activates some of the darkest things my imagination has ever come up with. Luckily I don't remember them, I just remember waking up terrified twice, one of them definitely screaming. Definitely had sleep paralysis a few times in that position too. I do remember one dream where I fell asleep with the back of my head on my couch armrest and it triggered a dream with my grandma in it a few years after she passed away. We were just chillin in her living room but she was there and it felt so real but instead of being happy to see her I remembered she was dead and started crying. I blame the sleeping position on that pain feeling so vivid. Certainly just as vivid as when I was at her funeral, but the despair and pain came back out of nowhere that night. I feel like if I would've had the dream while sleeping in a different position the feelings of pain/sadness wouldn't have hit at all, maybe would've just had good memories.