Wait. You could be tired even before the practice begins, couldn't you? Besides, how to know that it's the real feeling instead something we imagine in order to play save?
Here's is what you can ask yourself before you stop:
Have I discovered something that I didn't know before about this piece, composer, organ, myself, others, or life, during practice?
If not, practice a little more. That's how you will forge the will of steel.
(Amazingly, often the tiredness can be postponed quite a bit with this attitude).