Coding is not an assembly line. If you're too tired when you do it, you move more slowly, and you make more mistakes. You then have to find and fix the mistakes, which is even harder when you're tired.

Extreme Programming (XP) is about going absolutely as fast as you can. Two of XP's maxims are "Quit when you're tired" and "Never work overtime for more than one week in a row" - because you go faster when you quit when you're tired and don't work tons of overtime.