A human’s ability to do several things at once is a wonder of biology: it means we can eat a beef patty, while walking down the street, listening to music and daydreaming about the weekend all at the same time. But some kinds of multitasking costs you more time than you save.

Doing two things at once, like singing while you take a shower, is not the same as instant messaging while writing a research report. Don’t fool yourself into thinking you can multitask jobs that need your full attention. What you are doing is shifting your attention back and forth between the activities quickly — you’re juggling. When you juggle tasks, your schoolwork suffers and takes longer.

When your brain switches its attention from one task to another, it takes time to get into a new train of thought. You lose any momentum you had on the first task, which costs you on the next switch.

On the Internet where distractions abound, switching tasks can cost hours. A recent study showed that students who were interrupted, while they worked took an average of 25 minutes to get back to what they started.

If you’ve got work that requires engaged thinking — like reading, writing, or even just a serious phone call, stop juggling and start single-tasking.

For example, if you’ve got a dozen e-mails to answer and presentation slides to prepare, complete the slides before you look at the e-mail. You’ll get both jobs done faster than the juggler, who switches between the email and the slides every few minutes.

You already know that some kinds of multitasking can be hazardous to your health, like texting while driving. When it comes to splitting your attention between tasks, remember the difference between multitasking and juggling.

When you have the choice, stop juggling and get things done faster — one at a time.