What does this joke mean (may be offensive)?

So the routine goes like this:

The comic pretty much made a joke about people that say they’re bad test takers, to which he replied, “you mean you’re stupid….the part where we find out what you know?”

“Oh yeah….I’m a brilliant painter, minus my god-awful brush strokes..”

”...how the paintings crystallize up here (pointing to his head), but as soon as paint touches canvas, I develop Parkinson’s…”

Then he says:
“I apologize if there’s any parkinson’s painters here, I assume your best work is in the A.M….”

What does that mean? “I assume your best work is in the A.M.?