Sometimes while sleeping, I feel as if I can't move no matter how hard I try. It's like I'm stuck. What is that and what causes it?

Sleep paralysis - Wikipedia, the free encyclopedia

This might describe some or all of your experience. Does any of this sound familiar.

Sleep paralysis. It is very common, more common then people think.

I was just going to answer this and I see that jesushelper said the same thing I was going to say.