[array_key_first]

Because it's not linear but there's still an equation. It's not arbitrary. Meaning, 1 day of work is obviously less productive than 4. But 4 might not be less productive than 5.

It's also not about half-assing. If you work someone 80 hours a week, and they work as hard as they possibly can, they will still likely be less productive than if they just worked 40 hours. Because they start making mistakes, which they then have to fix, and make more mistakes while they fix those mistakes...

After a certain point, your productivity might be negative. Meaning, no employee at all might be more productive than an employee working 100 hours a week.