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How hard is it to stay retired? As you've no doubt heard, football legend Brett Favre just came out of retirement -- for the second time. But it isn't just sports figures who see retirement as little more than a passing phase: Polls suggest that anywhere from one-half to three-quarters of working Americans plan to return to some sort of work after they retire -- that is, if they expect to retire at all. Some can't imagine life without some kind of work; others simply need the money. For many people, especially Gen-Xers, the notion of working after "retirement" may almost seem a given, especially for those who are struggling to save enough in the current recession.

But as it turns out, unretirement can have as many complications as retirement. There's a giant gap between what people say they're going to do after retirement and what they actually do: A 2009 study by the Employee Benefit Research Institute (EBRI) found that 72 percent of workers planned to work after "retirement" -- up from 66 percent in 2007. But in fact, only 34 percent of retirees said they'd actually gone to work at some point during retirement. (A recent poll by the Longevity Alliance, conducted by Harris Interactive, found much lower percentages for both those who planned to work and those who actually did, but the gap between what people said and what they did was still there.)

There's a similar gap -- if not quite as dramatic -- between the age at which people expect they'll retire and the age when they actually do. While many say that the current economic mess has led them to delay retirement, the effect of these planned delays is hard to find in the data. As EBRI notes, among the people who have changed their expected retirement age within the past year

Why is this? Well, when you get older, to borrow a euphemism from Donald Rumsfeld, stuff happens. You may have health problems that keep you from working; getting a job may be harder than you thought. Heck, you might even decide that a life without paid work isn't quite as boring as you thought it would be. If you've already started getting Social Security, you may face reductions in your benefits if you go back to work (though this is only the case if you started your benefits before you reached your "full retirement" age). The AARP has a very useful page that can help you to sort out some of the costs and benefits of going back to work. And you'll find some helpful advice in this CNNMoney story.

You shouldn't count on being able to work enough in retirement to make up for a significant lack of retirement savings. But if you're healthy enough to work, the benefits of working in some capacity after you hit retirement age can be considerable. I suspect the percentage of those who actually do work after the age of 65 -- as opposed to just saying they will -- will increase considerably as the boomers and then the Gen-Xers hit that age.

Are you planning to work after the age of 65? If you're already past that age, are you working, or do you plan to return to work at some point in the future? If so, why? Money? Self-fulfillment? Health care benefits?