I had been having symptoms of Restless Legs Syndrome (RLS) for several years and had complained about it to my general neurologist when he informed me that Vitamin-D deficiency can cause those symptoms. As an RN, I knew that low potassium could cause muscle cramps in the legs, but not Vitamin D. He checked by my blood levels and mine was significantly low. Long story short, I started supplementation and he kept checking my blood levels as I titrated up until my serum levels reached the "normal" range (30-100), when my symptoms went away completely. I haven't had any more problems with RLS except for the one time I ran out of Vitamin D and stopped taking it for about 10 days. I take 5000 IU per day in order to keep my serum level at about 40-45, but everyone's needs will vary.