You're not bipolar, so what are you being treated for...do you have symptoms? Lithium is used to treat mania. If you don't have mania, it's no wonder you feel sluggish on lithium.
Lamictal is generally very well tolerated and has very few side effects. I find it to be a good med. If my dose is too high I feel "flat."
I cannot sleep without taking a sleeping aid - but then I have BD.