After 50 hours a week, your error rate goes way up. @oscarkelly is right about setting limits, but people forget that sloppy overtime practice can actually make your skills worse over time.
What about the muscle memory side of things? When you're dead tired and keep practicing a skill wrong, you're basically training your body to make mistakes. It's like your brain starts to accept the sloppy version as the right way to do it. So you're not just making more errors now, you're actually building bad habits that are way harder to fix later. That's how people burn out and lose their edge for good.
Overtime's a trap if you're not careful. The extra money looks good until you're too fried to think straight on normal shifts. Set a hard limit on how much you take on, or you'll pay for it later in lost skill.