A human's ability to do several things at once is a wonder of biology: it means we can eat a burrito while we walk down the street and listen to music and daydream 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. You're not really having a conversation while you write; you're shifting your attention back and forth between the two activities quickly. You're juggling. When you juggle tasks, your work suffers AND takes longer--because switching tasks costs.
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 or in an office where distractions abound, switching tasks can cost hours. A recent study showed that office employees 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 emails to answer and presentation slides to prepare, complete the slides before you look at the email. 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 or blow-drying your hair while you're in the bathtub. 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.