Sal, the most common cause is an electrolyte disturbance (sodium, potassium, magnesium, etc.), but it might also be due to a neurological problem or a problem with the blood flow in the legs. You can try taking a little extra salt and something like Slow K® and Slow Mag® for a week or so and see if it clears up. If it doesn’t, you will have to see your doctor. Good luck.