> Surely everyone gets some paid vacation time.

Not in the US.

https://www.bls.gov/opub/ted/2017/mobile/73-percent-of-all-c...

Also technically receiving paid vacation and actually being able to use it are often not the same: in retail us at the bottom of ten end up cashing out vacation at the rate of a few hours a week to compensate for getting fewer hours a week in the slow season but still have enough days scheduled (or are functionally on call) that we can't leave town.

I work for myself. Even sleep has an opportunity cost.

Marginal utility of time spent working usually declines.

That is to say that time off will pay off in increased productivity.

Another way to say it, insufficient rest has its own cost.

15 days per year, and that's quite lot compared to most previous jobs I've had.