Sam I agree with Dark Eyes; I also have never experienced fatigue on Lithium. However, if you are either taking a sleeping pill such as temazapam or a tranquilizer such as valium or Xanex; those two will definitely cause fatigue and lethargy but only if abused (i.e taking perhaps 2 more a day than precribed.)
Are you gettig enough sleep each night?
There for you, Maureen