When you work an insane number of hours, you’re going to get fatigued—for your neurons can only handle so much effort until they get worn out.
What’s happening here? Psychologists call it decision fatigue: the phenomenon where the more decisions you make, the worse at deciding you get. It’s part of the reason why judges give harsher rulings late in the day and hiring managers prefer the people they interview in the morning. Additionally, if you’re tired, you’re more likely to make unethical decisions—which suck and are to be avoided.