I have had severe leg (calf) cramps in the morning in bed. I found that one of the reasons was that I wasn't taking in enough water during the day. If I hydrate well, I get less cramps. But yes, magnesium really does alleviate them. That's one of the reasons Epsom Salts works so well. It's Magnesium (absorbed through your skin in the bath.) I take Magnesium tablets (which don't work that well for me, but probably because I'm not taking the right one. There are LOTS of different forms of it: https://www.vitacost.com/blog/vitamins-supplements/supplements/types-of-magnesium.html I use a product called "Natural Calm." It's a powder and it really does work. (You can get it at Walmart.) Though it's also acts as a diuretic, so best to just ask your doc before using. As Barbara said, "magnesium relaxes." It can also relax the bowels! lol
