Should I Be Concerned?

My cat Nestle, who is about 1 and a half years old, jumped up to sit with me here at the computer and I noticed he is shivering.  He has fallen asleep and is still shivering, but it has gotten a little less intense.  He hasn't been acting sick at all, he's his usual playful and silly self.  I have never noticed him trembling like this before though.  Just wandering if it's something I should be worried about or not.

Could be a lot of things vet attention is probably needed. My diabetic cat shook when her blood sugar got too low. I'd make sure you feed him if he does it again to assure its not the blood sugar. Absent health issues are you sure he is not purring and you just can't hear it? I know that sounds like a stupid question but I've thought there was something wrong with my shaky cat only to put my finger across her throat and realize she was purring but for some reason I couldn't hear it or feel it while petting. When you said it got less when napping it made me think that usually only when they are in deep sleep do they stop purring.