The fact is that during sleep, the activity of many organs decreases. Muscles also rest and relax. If, for some reason, the supply of blood to the leg muscles is insufficient, then the lack of oxygen in a horizontal position increases, which leads to seizures.
If a muscle cramps, it will be impossible to consciously relax it. The only way is to apply physical effort: straighten the toes with your hands or pull the toe of the foot towards you. After the cramp has passed, the limb can be massaged, this will help restore normal blood circulation.