Sleep Paralysis

It was just about the year 1985, I was a youthful Physical Therapy student. It was the hours of darkness and I went to nap. When I was slumbering on my bed, suddenly I felt something (like a mass of electricity) compressing on my back. I tried to move away by my legs and hands. What a wonder! It failed to move. It made me powerless on the bed. I really felt like loosing my consciousness. Then I tried to move with all my effort and resisted that compression and loss of consciousness. Fortunately it went away. I woke up and astonished. What is this!