As stated before by some, I've also read:
1. it can be that your matress is too hard OR too soft
2. it can be caused by eating too fatty foods or a lot of sugar
3. a sleeping disorder
4. Stress
5. Over working yourself at the gym
I had a similar problem a while back as discussed in this thread:
http://www.extremefitness.com/forum/...al-people.html
if you are currently not training I would look at some of the other things and try some changes- maybe see if it helps

good luck with everything!!