Checking e-mail while talking on the phone? Eating your lunch as you drive to a meeting? Reading this article on the treadmill? You’re not alone. According to a recent Scientific American poll, 90 percent of adults “multitask.” Doing two or more things at once seems an essential part of 21st-century life—but does it actually help us save time?

“Preliminary data suggest that multitasking may not make us as efficient as we once thought,” says psychiatrist Edward Norris, M.D., of Lehigh Valley Hospital and Health Network.

The fact is, switching back and forth between tasks may take more time than finishing one thing before starting the next. Researchers at the University of Michigan asked college students to alternate among various mental activities. Whenever the students switched between one thing and another, they worked more slowly.

The researchers found that it takes several tenths of a second to stop doing one task, decide to switch to another, then warm up mentally and get started on the new task. A fraction of a second might not seem like much, but it can add up as you repeatedly switch from writing your report to checking e-mail to answering the phone to ordering lunch.

Not just the speed, but also the quality of your work can suffer. For example, you might not really absorb what someone is telling you on the phone (even though you’re saying “uh huh” to all her comments) if you’re trying to do e-mail at the same time.

Multitasking can even be dangerous, if one of your tasks is cooking or driving.

In a recent survey, 31 percent of adults admitted they didn’t consistently wash their hands when switching tasks during meal preparation—a practice that increases the risks for food-borne illness. And researchers have found that drivers are much more likely to miss a traffic signal if they’re talking on a cell phone.

“As humans we have the special ability to multitask,” Norris says. “But it doesn’t come without a cost. We can learn to multitask more efficiently, but the stress of multitasking continues to affect our brains and bodies.”

Should you give up the habit? Not necessarily. You still may be able to save time by juggling, Norris says, if you choose your tasks wisely. In the Michigan study, the students lost the most time when their tasks were unfamiliar or complex. While simple things like walking or eating take almost no attention, you need to concentrate on a complex task like driving or balancing a checkbook.

The bottom line: You can easily chat on your cell phone while walking across town, but don’t try it when cooking dinner or helping with your child’s homework.