From WebMD

What causes muscle cramps?
The cause of muscle cramps isn't always known. Muscle cramps may be brought on by many conditions or activities, such as:
• Exercising, injury, or overuse of muscles.
• Pregnancy. Cramps may occur because of decreased amounts of minerals, such as calcium and magnesium, especially in the later months of pregnancy.
• Exposure to cold temperatures, especially to cold water.
• Other medical conditions, such as blood flow problems (peripheral arterial disease), kidney disease, thyroid disease, and multiple sclerosis.
• Standing on a hard surface for a long time, sitting for a long time, or putting your legs in awkward positions while you sleep.
• Not having enough potassium, calcium, and other minerals in your blood.
• Being dehydrated, which means that your body has lost too much fluid.
• Taking certain medicines, such as antipsychotics, birth control pills, diuretics, statins, and steroids.

Diabetics (and a surprising amount of the general population as well) are often magnesium deficient -- you might also try supplementing with chelated magnesium (NOT magnesium oxide!). Cramps are one of many possible effects.

In the six months or so before I was diagnosed, I would wake up nightly with the most excruciating leg cramps. I drank tons of water, and tried every supplement I could think of, all to no use. I had no idea at the time that it was a symptom of high blood glucose. As soon as I got my glucose levels under control, the cramping stopped and I have not had a single one in over a year.