The following is a quote from a Practical Peer-reviewed Journal for Practicing Physicians....

"No one knows for sure what causes nocturnal leg cramps. In many cases, there doesn't seem to be any specific trigger. However, sometimes the cramps are caused by overexertion of the muscles, structural disorders (eg, flat feet), standing on concrete, prolonged sitting, inappropriate leg positions while sedentary, or dehydration. Less common causes include diabetes, Parkinson's disease, hypoglycemia, anemia, thyroid and endocrine disorders, and use of some medications."

"To stave off future episodes of nocturnal leg cramps, consider the following tips:

Drink six to eight glasses of water daily. Doing so will help prevent dehydration, which may play a role in the cramping.
Stretch calves regularly throughout the day and at night. (See box below for more information.)
Ride a stationary bicycle for a few minutes before bedtime. This activity can help prevent cramps from developing during the night, especially if you do not get a lot of exercise during the day.
Keep blankets loose at the foot of the bed to prevent your toes and feet from pointing downward while you sleep.
Do aquatic exercises regularly during the week to help stretch and condition your muscles.
Wear proper foot gear. "

http://www.postgradmed.com/issues/20..._legcramps.htm