Recurring, terrifying sleep experience?

Sometimes, when I lie on the left, I fall asleep, but am still aware of where I am. I know I am asleep, it is completely dark like before you dream but I am...conscious, I start struggling. It almost feels like paralysis, but I know I am actually moving. It sounds ridiculous, but I have to physically open my eyes with my hands to wake myself up, it is very surreal and quite distressing. Has anybody else experienced this?