Why does this happen to me when I sleep? Very scary. ?

Sometimes when I sleep I wake up without opening my eyes because I cannot open them. No matter how hard I try to open them, move, or yell, I cannot. I try doing all of the latter until I fall asleep. While experiencing this, I feel as if, someone is on top of me keeping me from being able to move. This is a very scary and weird experience. I heard of other people feeling the same way and their explanations all seem superstitious. Has anyone experience this and or does anyone know why this happens?