Francis.. oh. my god. Did you have to go there?!
I often have nightmares where I wake up to find my cat on my chest.
Or even my pillow on my chest.
Though, putting your hands on your chest before you fall asleep does not guarantee that they will remain there to the stage of REM sleep that we have the deepest dreams at .
By the time we start to get into a deeper sleep, our bodies completely relax and sometimes we have muscle twitching.
Then there is a faze of REM sleep where your body is technically 'paralized' meaning, your muscles can not move on their own , and you are safe to have your deepest dreams with out your body reacting to what is going on in your head.
Each REM cycle is about 90 minutes in length and your body does not stay in one position that long. Circulation demands do not allow it to..
So I do not think that the position of your hands makes a difference unless they are on your chest during the REM phases. But if you start out with your hands on your chest, it is very VERY likely that they will not stay in that position long enough to effect your deepest dreams.