I have 2 bars of handsoap under my mattress cover in the lower half of my bed. Thanks to them, I’ve had no cramps for 2 years. I don’t know why this works but it does. Google it and you’ll find a forum and some attempts at a scientific explanation. It’s like the mustard, I guess. My brother swears by it. (It does zero for me.)
Not everything has to make sense. Give it a try.