I began to notice fatigue once half way back on the last flight of a looong day on our last of a 4 day. Enroute we were talking about how tired we were starting to feel and how long a day it had been. I was PF... night visual into atlanta... took over the radios after taxiing off and realized we were still on approach... neither of us caught it. Fatigue was definitely one of the factors. Lucky it was only a small problem. I am sure many of you have experienced far worse. But in this case we didn't even key in on it until mid flight.