This may be due to a lack of magnesium. Or it could be that your muscles are just so tense during the day that they hurt when you finally relax them at night. It could also be a symptom of Fibromyalgia - but you didn't mention if you hurt anywhere else and if you hurt during the day. I would try a multivitamin/multimineral supplement to see if perhaps you are missing something in your diet. - Please let us know how you fare.