Excerpted from Getting to 50/50: How Working Parents Can Have It All by Sharon Meers and Joanna Strober. Reprinted with permission from the publisher, Viva Editions, copyright 2013.

From the moment we get our first jobs, we're led to believe that more is better–more billable hours, more orders from customers, more time with clients, more meetings to set more goals, more tasks learned, more e-mails sent, more products produced, whether it's cherry pies or radial tires. More, more, more is the way of sending the signal that you're good, better, the best at what you do. Working all the time says you're tough, you're tops, you're macho (even if you're biologically female). The old way of being macho was to play basketball on a sprained ankle for hours on end. The new way of being macho is to work nights and weekends.

But competing when you're not at your best is always a mistake. "Businesses need to be 24/7," says Xerox CEO Anne Mulcahy. "Individuals don't."

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Not only do we not need to be "on" all the time, but for the good of our employers, we shouldn't be. It's no secret that when we work while exhausted, we often do more harm than good. Sleep experts have studied truck drivers and nuclear plant workers and have concluded that it is downright dangerous for them to work more than a six-to-eight-hour shift. The medical field is an infamous hotbed of sleep-deprived workers. Comparing hospital interns on a "normal" thirty-hour shift with those working shorter hours, one study found dramatic differences in performance. Interns on thirty-hour shifts misdiagnosed patients six times more often and made almost 60 per cent more errors compared with interns who got more normal amounts of sleep.

While most of us aren't doing jobs that could kill other people if we work past our peak, we can still do a lot of damage. Consider that e-mail you fired off to the Big Cheese at 11 last night (you misspelled his name), your after-hours voice mail to the supplier who's late again (you said something bad, but what was it?), being abrupt with the rookie who screwed up the numbers after working all night.

A guru to Fortune 500 CEOs told us, "If you can't get your job done in ten hours a day, there is something wrong with you." His point: No matter where you sit in the food chain there are only so many productive hours your brain can put in–after that, you'll make mistakes, you'll make a gaffe, you'll make a mess. "Your priority list should be short and very focused; you have to say no or delegate to others."

When faced with a need to expand capacity, we throw more people and more hours at the problem, rather than figure out how to work more productively. We call more meetings. We generate more e-mails. We get busy–but in the wrong way and for the wrong reasons.

Researchers know too much 24/7 causes serious problems, that workaholics, who incidentally develop stress-related ailments that drain employee health-care benefits, have compromised decision-making and problem-solving skills, as they become uncreative and forgetful. The Chernobyl disaster, the Exxon Valdez oil spill, and the explosion of the Challenger space shuttle were all the result of too much 24/7, according to Martin Moore-Ede, physiologist and author of The Twenty-Four-Hour Society.

Most bosses don't see the problem of pushing employees to the max because they get the results they want–short term, at least. The 24/7 ethic is a gross perversion of the good old-fashioned work ethic and it costs us a lot in productivity. In her book Finding Time, Harvard Business School's Leslie Perlow conducted a nine-month study of time management at a Fortune 500 firm. She found that "those who work hardest do not necessarily contribute the most to the corporation's productivity, and, in fact, that often no one benefits from this behavior, not even the corporation…if we had the incentive to get the work done in less time, we could create alternative ways of working that would be more efficient and effective."

If you do your job, and you do it well, you will still get ahead even if you don't get to the office at dawn and stay until your wastebasket is emptied. Your own biggest enemy may not be your boss, your meeting schedule, or the pinging sound of your e-mail inbox. Instead, it may be the little voice you're hearing that warns you'll be penalized for going against the grain–you know, pursuing that radical wish to see your kids for dinner.

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Sharon Meers leads Enterprise strategy for Magento, eBay's global commerce platform. Formerly a managing director at Goldman Sachs, where she worked for 16 years, Sharon and her husband founded the Partnership for Parity at Stanford to make the dialogue on these issues more fact-based, co-ed and fun.

Joanna Strober is the founder and CEO of an online company developing a platform and tools to help fight and prevent childhood obesity. Previously, she was a managing director at an investment management firm.