Many things about becoming a parent surprised me. Sure, there was the obvious, like exactly how many days in a row one can survive on less than three consecutive hours of sleep. Or the number of onesies that can get thrown up on in 24 hours.

But there were work-related surprises, too. As I got into the groove of working parenthood, I was pleasantly astonished to see my efficiency skyrocket. With strict limits on my time (e.g., daycare closing at 5:45pm and the risk of being charged $10/minute for being late), I developed a laser focus on priorities.

I started a habit of beginning each day writing a Post-It note containing my three top objectives and zoomed in on accomplishing those. I spent my time at the office and in meetings more productively than ever. And I learned quickly that certain things—like office gossip—had to go out the window. There is truly no time for drama when you’re a working mama.

It turns out I’m not alone in this experience. As I’ve worked both with my fellow attorneys in the legal sector and with new working moms from all different industries through Mindful Return, I’ve discovered that new powers of efficiency and productivity do tend to come with the terrain. Many working parents, it turns out, cite prioritization and efficiency among their newfound leadership superpowers.

Enter the sad reality, though, that many modern workplaces continue to reward hours worked over impact, efficiency and effectiveness. Either by rewarding an employee’s number of billable hours or through the proverbial hours of “butt in a chair,” the message these workplaces send is: quantity over quality.

Why? Probably a combination of history and laziness.

The standard, American eight-hour workday is a vestige of the industrial revolution’s efforts to reduce the number of hours of manual labor for factory workers. Yes, this was a good thing a few hundred years ago, but it doesn’t exactly fit with today’s workplace culture. And the “ideal worker” standard that describes the dutiful man in the office from sunup to sundown that Brigid Schulte writes about so eloquently in Overwhelmed: How to Work, Love, and Play When No One Has the Time is a vestige of a bygone era, too.

New research confirms that it’s simply not possible to be productive for eight hours per day.

“Turns out, the secret to retaining the highest level of productivity over the span of a workday is not working longer—but working smarter with frequent breaks,” wrote Julia Gifford in The Muse when she posted the results of a time-tracking and productivity study.

I’ve definitely experienced this phenomenon for myself. Some of my most productive work days have come when I’ve committed to using the Pomodoro technique of working with breaks at scheduled intervals.

And the so-called “split shift”—which I use daily—re-energizes me for some evening work, after I’ve gone completely screen-free over dinner and the bedtime routine with my family.

Employers, take note. If you want to retain more women, and if you value the working parents at your office, think hard about how you evaluate them.

Yes, it’s more difficult to measure output, productivity and effectiveness than it is to measure the number of hours someone spent sitting at a desk or working on a particular project.

Yes, it’s harder in some industries to figure out how to price a project with a flat fee, rather than by billing that client by the hour. But just because something is more challenging doesn’t mean it’s not worth doing.

By rewarding output over time, and by ceasing to punish star employees for their efficiency, you stand a much better chance of increasing the number of women who want to stick around and lead your organization.