As anyone who has ever gone to the gym can tell you there are two components to a workout's difficulty: intensity (how heavy is the weight) and duration (are you lifting it five times or 20).

Might the same be true of work? Can you find the sweet spot of professional happiness by reducing duration but amping up intensity? 

Taking it easy time-wise at work is having a bit of a moment. High-octane professionals from Metalab founder Andrew Wilkinson to developer Kyle Bragger have recently come out and confessed that they've dialed down the intensity on their workweeks with positive results. These entrepreneurs say they benefited from trading their 80-hour marathons for sub-40 hour sprints, but if you're thinking of following in their footsteps, don't confuse reducing the duration of your work with reducing the difficulty of the problems you're working on.

Why? Tackling difficult problems is one of the keys to professional satisfaction, according to Rosabeth Moss Kanter, a professor at Harvard Business School and author of the book Evolve! In a recent HBR Blog Network post she reveals some of the conclusions she reached researching her book and claims that happiness often correlates with work on truly difficult problems:

The happiest people I know are dedicated to dealing with the most difficult problems. Turning around inner city schools. Finding solutions to homelessness or unsafe drinking water. Supporting children with terminal illnesses. They face the seemingly worst of the world with a conviction that they can do something about it and serve others….

I see that same spirit in business teams creating new initiatives that they believe in. Gillette's Himalayan project team took on the challenge of changing the way men shave in India, where the common practice of barbers using rusty blades broken in two caused bloody infections. A team member who initially didn't want to leave Boston for India found it his most inspiring assignment. Similarly, Procter & Gamble's Pampers team in Nigeria find happiness facing the problem of infant mortality and devising solutions, such as mobile clinics that sent a physician and two nurses to areas lacking access to health care.

Moss Kanter concedes that, "of course, daunting challenges can be demoralizing at times," and notes that progress towards tough goals is not linear, but she feels that these highs and lows average out to a much higher level of professional happiness overall compared with simply kicking back and not stretching yourself at work.

So could these two insights be combined? Could both the fed up founders curbing their workweeks and the professor advocating pushing the boundaries of your ability at work be correct? Perhaps the secret of a healthy professional routine isn't all that different from the secret to a successful health regime. Give yourself challenging goals for motivation but also make the time commitment sustainable so you can keep it up.

Do you agree that solving difficult problems is at the heart of professional happiness?