Effexor and tiredness?

I have been on Effexor XR for anxiety attacks for a few months now and feel tired if I don't have a nap in the afternoon. I try to stay awake but seem to nod off easily. This past week I was studying for an exam which I had later that day and almost missed my exam because I dozed off. Just wondering if anyone has been successful in lowering their dose for tiredness? I am currently on 75mg and would like to go to 37.5