The most challenging task you'll ever face as an investor is figuring out how much money you need to save for your retirement. There's good news, though: you may not need to set aside as much money as you might have thought to retire comfortably.

What you'll actually need
When you're saving, it's easiest to focus on how much to set aside and where to invest. But the flip side of retirement planning is at least as important: how much you plan to spend when you retire. And that's where some of the assumptions that financial planners make start to come into question.

One study actually took a look at what people spend during retirement. What the study found is that as people grow older, they tend to spend less on just about everything, including entertainment, food, and transportation. The only category of expenses that didn't drop was health care, as you'd probably expect. Yet with Medicare and other programs in place to backstop seniors, even those expenses didn't rise as much over time as you might expect.

Setting the context
The expense study has huge implications for retirement savers. Many who may have thought themselves unable to save successfully for retirement may now be able to do so.

In recent years, prospects for retirement have seemed ever more frightening. Those who are close to retirement and have substantial assets set aside have seen their value plummet in the market's meltdown, and although the rally has restored some of those portfolios closer to full strength, the latest correction has everyone back on their toes again.

Meanwhile, younger investors face the prospect of having to rely solely on their own financial resources to support themselves after they retire. Company pensions are nearly a thing of the past, while even Social Security has issues that may well be resolved unfavorably for those with decades to go before reaching retirement age.

In that context, it's easy to understand why many financial planners recommend that people set aside as much as they can toward their retirement. Given so much uncertainty, they argue, it's better to be safe than sorry. And with many people living long past 65, you have to plan for many contingencies if you want to be certain that you'll never run out of money.

Set the right goals
What the study shouldn't do, however, is give you a false sense of security. To set yourself up on the right track to take advantage of potentially lower expenses, you have to get your financial house in order. Some expenses will naturally go away, such as supporting your kids. But you still have to make sure you eliminate short-term debt, get your mortgage paid off, and make allowances for things like long-term care and Medicare supplemental insurance before you retire.

What it also means is that you may have more latitude to be less aggressive in your investing. Many late-starters feel they have to take on big risks by picking swing-for-the-fences stocks. They might have picked Dendreon (Nasdaq: DNDN) in the hopes that it would indeed get its long-awaited approval for its cancer-fighting drug. They might have gambled on the building-intensive Las Vegas Sands (NYSE: LVS) and MGM Mirage (NYSE: MGM) successfully navigating themselves out of the shoals of potential bankruptcy during 2008's credit crunch. Those bets paid off, but can you count on being as successful for the rest of your investing career?

With less pressure to maximize your returns, you can build a more balanced portfolio. For instance, neither the iShares DJ Select Dividend Index ETF (NYSE: DVY) nor the iShares Investment Grade Corporate Bond ETF (NYSE: LQD) are going to have your portfolio hitting it out of the park. But they're both valuable resources for diversification and can give you a much smoother ride along the way than you'd get from riskier stocks.

Get it done
Of course, if you've already saved enough to cover all your contingencies, then the prospect of potentially falling expenses makes you even more secure. But the most important thing to take from this news is that you have no excuses not to start saving for retirement. Even if you think you're too late, every dollar you save now will get you a bit further toward your goals -- and those goals are closer than you think.

Tune in every Monday and Wednesday for Dan's columns on retirement, investing, and personal finance.