Huge. When I'm sleepy my error rate can go up by as much as 300%. It's terrifying to think of what that means in terms of how it must be impacting by work when I see that happen. I often study in the morning before work, and if I can see I'm not performing well, I know to do yoga/exercise/etc before I try to do any productive work. (I found that my study outcomes were nearly always better after exercise, if I started badly)