This has only occurred twice to my knowledge, but both times were scary.
I'm going to start with a bit of my background first. At the age of ten, I'd lived in a different home from my mother. She was a single mom who did the best she could to raise us and avoid involvement with DCFS, so my sisters and I were separated and sent to different family members. My older sister stayed with my mom, my younger sister with my oldest aunt, and myself to my youngest aunt. My mother lived in a house split into two apartments.
Here, my experiences start.
I went to stay the night with her and my older sister one night. As my mother had just moved in and hadn't gotten furniture yet, I slept on the floor of her room. She was working third shift at a hotel, so it was just my older sister and I. My sister was in the living room, watching television, and I had gone into my mother's room to go to sleep. Just as I was falling asleep, I began to feel as though I were sinking through my pillow and the floor. At once, I felt something surrounding me as I sank. I couldn't move, but my eyes were open. The scariest part was that I could actually SEE myself sinking! Before I knew it, I could move again. I ran into the living room to my sister, and slept in there that night.
A little over ten years later, I found myself in the same situation. It was a week before I was due to be baptized at this church I had joined, and I was staying at my older sister's place for the weekend. I was settled on the couch, ready to fall asleep, when I felt that familiar paralysis and sinking feeling. My mother entered the living room and began to watch the television. I couldn't cry out to her! I even felt hands wrap around my throat and begin to choke me! I couldn't breathe, and I frantically began to pray for help from God in my head. Shortly after I began praying, I felt myself become freed from the attack.
I haven't had that sensation since, but I fear it could happen again at any time.