Adderall making me tired but still working?

So I have been on adderall for 2-3 months and it has worked perfectly for the most part, but recently it has began to make me tired. I take my medication around 5:30 every morning and by 12 I begin to feel tired but I still am able to stay focused on my tasks. I become a lot more tired as the day goes on and I basically just want to sleep by 6. Has anyone else had this happen or know how to fix it?

I am on 20mg XR and I don't have another check up with my doctor for another 2 months, so i'm trying to get some quick help here. Thanks!