Ever had a nightmare which effected like living horror to you, you're trying hard to scream for help or shout a prayer but you felt like freezed?
Believe it or not, your body is virtually paralyzed during your sleep - most likely to prevent your body from acting out aspects of your dreams. According to the Wikipedia article on dreaming, “Glands begin to secrete a hormone that helps induce sleep and neurons send signals to the spinal cord which cause the body to relax and later become essentially paralyzed.”