Most Americans work impossibly hard. We put in long hours and maximum effort, but better performance often eludes us. I’m no exception. I remember being in my 20s and landing my dream job as a management consultant at the posh London office of the U.S.-based Boston Consulting Group. I strode through the front doors on my first day wearing an elegant new blue suit and equipped with what I thought was a brilliant strategy for impressing my bosses: I would work crazy hours.

Over the next three years, I toiled for 60, 70, 80,...