My wife has been bothered by this for years, so we've researched the hell out of it and tried about everything. There is no agreement as to cause. Most doctors say they simply have no idea.
However . . .
Pulling out the sheets and blankets from the foot of the bed definitely helps.
Taking cal/mag supplements totallying 1500mg calcium, 750mg magnesium definitely helps.
Taking more magnesium than that can give a person the runs.
Salt is an issue. If you eat a natural foods diet, i.e. you cook from scratch, add up your daily salt intake. It must be at least 3/4 tsp./day, which it may take a little work to achieve. This is for no exercise. Add to that the salt you need to take in to combat salt losses due to exercise. Endurolytes during exercise worked best for her. We didn't increase our base daily intake to take exercise into account. We both have low BP.
Doing the salt calculation fixed 90% of the remainder of her cramping problem. It's still a problem, but it's minor. Maybe we need to increase salt a little more, but we like how our food tastes as it is.