????In an uncertain job market, fortunate people with steady paying gigs often think twice before shutting down for the day. But is\ there any benefit to responding to those additional emails or hanging around the office for that extra hour? Is there a weekly hour sweet spot?

????It's a tricky question. One hour per week is too little to do a job well, and 168 -- the total number of hours in a week -- isn't right either. Such a sleep-deprived person would be non-functional (not to mention smelly from a lack of showering).

????The answer must be somewhere in the middle; a point of diminishing returns where previous hours boost productivity by a large margin, and additional work hours wouldn't help much. But what amount of hours could that be?

????In a competitive economy where people still value their family and leisure time, figuring out the answer could give businesses a strategic advantage, so it's no surprise that plenty of people have conflicting opinions on the matter.

????New York City Mayor Michael Bloomberg tells graduates in his usual commencement address that "it never hurts to be the first one in in the morning -- and the last one to leave."

????In last year's bestselling book Rework, 37 Signals co-founders Jason Fried and David Heinemeier Hansson complain about people who "try to fix problems by throwing sheer hours at them.... This results in inelegant solutions." Workaholics "aren't heroes," they write. "They don't save the day, they just use it up. The real hero is already home because she figured out a faster way to get things done."

So, what's the magic number?

????Bloomberg, Fried and Hansson are all successful people, so who's right? Up until now, there hasn't been too much data surrounding this question, but researchers at Harvard Business School, the London School of Economics and other institutions have recently begun an ongoing CEO Time Use Project to figure out exactly how work hours relate to success. Using time logs kept by CEOs' personal assistants, and looking across different cultures, the study asks how CEO time use corresponds with a company's performance.

????At this point, data is only available from a group of Italian CEOs of large firms. But according to Harvard's Raffaella Sadun, "we found this very strong correlation between the number of hours spent at work by a CEO and the productivity of the firm" (defined as revenue per employee) "and also the profitability of the firm." Every one percentage point rise in hours worked meant firm productivity rose by 2.14 percentage points.

????Does that mean Bloomberg is right and we should all work more hours?

????Not necessarily. First, Sadun and her colleagues found a big difference in productivity based on how a CEO spent those additional hours. Meeting with employees correlated with more productivity. Meeting with consultants or other outsiders did not.

????And the Italian CEOs didn't turn out to be working what many executives would consider a taxing workweek. Each additional hour boosted performance, but that's not too surprising given that the average CEO in the study was only logging 48 hours per week.

????Granted, these were only working hours that the personal assistants knew about, and so, to a degree, "we don't get people who worked over the weekend or worked at home," says Sadun.

????But an assistant would know about a CEO's weekend or evening conference call, a dinner or a speech. "I don't want to say Italians are lazy," says Sadun, who is Italian. Tentative analysis of American CEOs shows they may be working a few more hours. But keep in mind that these Italians working 48 hours a week were successful enough to be leading large companies in a competitive world. As a group, it's unlikely that they would be leaving a full 20, 30 or 40 productive hours on the table, given the stakes. That implies that perhaps the point of diminishing returns is not too far above that.

????There's evidence this may be true in the U.S. as well, even when people claim to be working longer hours. A dirty little secret of time use research is that people lie, a lot, which is one reason Sadun and her team rely on assistants' records rather than self-reported time use. One analysis comparing estimated workweeks with time diaries conducted by sociologist John Robinson of the University of Maryland found that the average person claiming to work 70, 80 or more hours per week was logging less than 60.

How valuable are the extra hours?

????Even 60 hours may be past the point of diminishing returns for some people, according to Cara France, CEO of Sage Consulting Associates, a 65-person consulting firm based in the San Francisco area.

????France's goal is to work 40 hours a week or less, in part to spend time with her 5-year-old twins. She calculates 40 hours as almost exactly her point of diminishing returns.

????"If you work 65 hours vs. work 40 hours, will you get more done? Yes," she says. "But will you get 25 more hours done? No. Will you get 10 more hours? Maybe."

????But even that isn't certain, she says. Having worked longer hours in past professional projects, "I'd say the diminishment is gargantuan."

????David Lassman, vice president of operations at Leed's, which makes promotional products, agrees. He works a 9 to 10-hour day, and expects the same from his direct reports. The last hour or two is "just straight good productivity -- you get more from it," he says. "After about 10 hours, I start to lose my edge. My thought processes aren't as sharp."

The temptations of an endless workday

????You've seen it too -- people order food and flit from cubicle to cubicle, clearly not doing anything productive. Or else they go home, but leave the Blackberry on all night, sneaking a quick check when they get up to go to the bathroom, even though the chances of anything important appearing on the screen are roughly nil. It's all too easy to breeze past the point of diminishing returns when you don't even have to go into an office.

????Christa Carone, chief marketing officer at Xerox (XRX), recites a quote to that effect: "Work is no longer a place; it's a state of mind," she says in an email. "It's become less about when I turn off the office lights and more about when I turn off (at least, mentally) the inbox."

????This phenomenon has more to do with psychology than rational economics. In a competitive world where jobs aren't certain, it's tempting to disregard the rational approach out of fear of missing out.

????Melissa Gerstein is part of a team of three broadcast journalists launching Moms and The City, a show that airs on NBC in a few cities and also in segments on New York City's Taxi TV. Her schedule -- up at 5:30 a.m. and returning emails into the wee hours --- is grueling, but she worries she may still be working below the point of diminishing returns.

????"When you're an emerging brand, it's almost impossible to turn things down," Gerstein says. "You want to take advantage of every opportunity that presents itself to you."

????That means she and her business partners check email and make calls while at ballet lessons or pushing strollers down the street. Though the rise of new media outlets has created opportunities to promote a new show like hers, sometimes, "I get overwhelmed…. Sometimes I miss the good old days when it was a lot simpler," she says.

????She's not alone, but others attempt to recreate that simpler world, even if no one else does. Cara France, for instance, doesn't check email after work. "When I'm on, I'm on, and when I'm off, I'm off," she says. So far, she's happy with what that means for her business. Sage doubled its revenue from 2009 to 2010 and recently made it onto Inc's list of the 5000 fastest growing private companies. Sounds like her time was well spent.