Another possibility. I agree with dr. Gordon, but would add another possibility: chronic venous insufficiency. Many docs & patients are unaware of the association between chronic superficial venous insufficiency (faulty valves leading to increased pooling/pressure in leg veins) and chronic leg cramps. A board-certified phlebologist (vein specialist) can evaluate such issues. Minimally-invasive procedures may help.
Good stretching. First try stretching before you go to bed. Stand 2 feet from a wall. Plant your feet fully and flat on the floor. Lean on the wall for 120 seconds with your shoulders. This stretches out and relaxes the muscles on the back of your calves.