I once worked with a guy who didn't take vacation weeks. Instead, he took vacation days

Just Fridays. Twenty a year. Counting certain holidays, he worked four-day weeks for nearly half the year. 

Unusual? Sure. But here's where it gets interesting: On his five-day weeks, he worked at a normal pace.

On four-day weeks, his Wednesdays and Thursdays were different. He avoided certain meetings. He bailed on certain calls. He didn't linger, didn't gossip, didn't spend any "bonding" time with other employees: On those days, he kicked ass.

He didn't want to spend the Monday after a short week having to catch up for missed time--so he made sure he was as productive in those four days as he normally was in five.

While my coworker's motivation may have been different, the outcome was the same as that of a widely publicized experiment conducted by Perpetual Guardian, a New Zealand trust management company. Perpetual trialed a program where they paid employees their regular salary while only requiring four-day workweeks.

The result? A surprising 20 percent gain in employee productivity, and a not-so-surprising 45 percent increase in self-reported employee work-life balance. Eventually, the company made the policy permanent.

And why not? After all, why is a five-day workweek the norm? As Adam Grant points out, the five-day workweek is a human invention. Same for the eight-hour workday.

It's arbitrary. Sure, there are reasons for the five-day workweek, but mostly it's just a longstanding practice.

Some have started to experiment with shorter workweeks. ZipRecruiter says the number of job postings that mention four-day workweeks is up 300 percent over the last three years. Even governments are dipping a toe in the shorter workweek waters: Spain is considering a proposal to encourage companies to offer a four-day week.

But wait, there's (not really) more: While four-day workweek job postings are definitely up, they still make up less than 1 percent of all job postings.

As for the Spanish proposal, it includes subsidizing companies that offer four-day workweeks, assumedly to offset lost productivity. (If you're an employee trying to convince your boss to go to four-day workweeks, maybe leave that tidbit off your list of justifications.)

Why? Because most leaders still manage by hours worked, not by results.

Like Bill Gates, who memorized employee license plates so he could look in the parking lot to see when people got to work, and when they left. 

Eventually he realized that managing by results was much more effective, and a much better use of his time. (That's especially true where remote work is concerned; ask anyone who has learned how to game their Slack status dot.)

Want to experiment with four-day workweeks? Want your employer to experiment with four-day workweeks?

First, take a step back and create meaningful performance metrics. Set meaningful targets for tasks, projects, and deliverables. Determine what needs to get done. 

Start managing by outcomes, not hours. Because to make it work, your employees will definitely have to increase per-hour productivity. (Working less hours at the same output level will simply mean less gets done, not the same. And definitely not more.)

Consistently manage by outcomes and your employees will then have a genuine incentive to be more efficient and effective while working. If I know I get Mondays off as long as I work hard every Tuesday through Friday, I'm a lot more likely to kick ass.

Then, showing the flag by putting in long hours--yet accomplishing relatively little--won't be necessary. Being at work, but not working, won't be necessary.

Because those behaviors won't be rewarded. 

Your employees aren't productive just because you expect them to work five days a week. (Nor are they productive just because you expect them to work 10- or 12-hour days.)

Like butts in seats, days or hours worked is a terrible proxy for productivity.

Decide what needs to get done. Give people the freedom, authority, and most importantly the focused time--by cutting out unnecessary meetings, calls, etc.--to get those things done.

Only when desired outcomes are clear, and people are given the means to achieve those outcomes, will they not need to work as many hours. That's the only way they'll be more engaged. More efficient. More refreshed, recharged, and happier about their work-life balance.

Which will make them even more engaged and productive.