When you promised to take your friend out to lunch a few weeks ago, it seemed like there'd be no problem carving a few hours out of your day. But, as that lunch date rolls around, all of a sudden you find yourself pressed for time, barely able to stay long enough to scarf down your food. Everyone overcommits, whether it's taking on one too many projects at work or promising to help at your child's bake sale. Two business professors from Duke University and the University of North Carolina–Chapel Hill took a closer look at this phenomenon, trying to figure out why we make promises we can't keep.

What the researchers wanted to know: Why are people likely to overbook themselves for commitments in the near future?

What they did: The researchers gave seven different surveys to students at Duke, the University of North Carolina, and the University of South Carolina. The surveys asked different types of questions, but all were designed to figure out whether students would be more likely to commit to spending time or money more easily if the expenditure was now or several weeks in the future. For example, one questionnaire asked students if they would be more likely to volunteer several hours at a local charity the next day or in two weeks.

What they found: People tended to think that they would have more time in the future than they have today. That makes people commit to things down the road that they may not be willing to do if the commitments were more immediate, say the authors, and leaves people feeling pressed for time after they've promised to do something they can't fit into their schedules. "People are consistently surprised to be so busy today," write the authors. They said that because people's days are often variable—with different activities filling up each day—it is hard for people to predict how much time they will have in the future, leading to overcommitment. By contrast, most people are less likely to spend more in the future than they are today, presumably because a person's flow of money remains more stable.

What it means to you: If you find yourself rushing to fit in everything in your appointment book, you might want to ask yourself the next time you make a commitment if it really, no really, will fit into your schedule. Since most people tend to think they'll have more time in the future than they actually end up having, it's worth giving a second thought to dates, work projects, or appointments you make.

Caveats: Most of this study was done using college students, who sometimes have different perceptions of money and time management than the rest of us.

Find out more: Overcommitting leads to stress. The National Mental Health Association has tips on how to relax.

Read the article: Zauberman, G. and Lynch, J. G. "Resource Slack and Propensity to Discount Delayed Investments of Time Versus Money." Journal of Experimental Psychology: General. February 2005, Vol. 134, No. 1, pp. 23–37.