Thanks so much for the praise, K P!
And thanks for the questions. I might research them a bit more and eventually write an article :-)
Meanwhile, here's what I know.
From what I've read, people seem to be more likely to suffer nightmares when sleeping on their back, which I assume you're doing when you're sleeping with your hands on your chest. Particularly hypnagogic hallucinations and sleep paralysis are more likely. I wrote about it here:
But as far as I know, no one really knows why sleeping on one's back increases the likelihood of bad dreams.
As to nocturnal leg cramps, I've suffered them myself, mostly in the summer when it's hot. My understanding is that when you sweat a lot--due to exercise or hot weather--you deplete your body's minerals (particularly potassium) and that makes cramps more likely. Although there seem to be other causes as well, such as certain medications, pinched nerves, and general dehydration.
If I get these nocturnal cramps, they usually subside quickly if I manage to get out of bed and stand up.