Because the brain cannot fully focus when multitasking, people take longer to complete tasks and are predisposed to error. When people attempt to complete many tasks at one time, “or [alternate] rapidly between them, errors go way up and it takes far longer—often double the time or more—to get the jobs done than if they were done sequentially,” states Meyer . This is largely because “the brain is compelled to restart and refocus” . A study by Meyer and David Kieras found that in the interim between each exchange, the brain makes no progress whatsoever. Therefore, multitasking people not only perform each task less suitably, but lose time in the process. [wikipedia]
For me, I multitask when I'm bored. Typically, I find myself losing interest in the task at hand so I jump to another task on my list. Then I feel guilty for orphaning what I had been working on so I'll jump back.
More Wikipedia you say? You got it!
When presented with much information, the brain is forced to pause and refocus continuously as one switches between tasks . Realistically, this is “a rapid toggling among tasks rather than simultaneous processing.” According to a study done by Jordan Grafman, chief of the cognitive neuroscience section at the National Institute of Neurological Disorders and Stroke, “the most anterior part [of the brain] allows [a person] to leave something when it’s incomplete and return to the same place and continue from there,” while Broadman’s Area 10, a part of the brain’s frontal lobes, is important for establishing and attaining long term goals . Focusing on multiple dissimilar tasks at once forces the brain to process all activity in its anterior. Though the brain is complex and can perform a myriad of tasks, it cannot multitask well. [wikipedia]
Last year there was an article in the Standford paper dealing with the topic of multitasking. As you could expect, the outlook was grim for those that boast about being able to - well you know.
"By doing less, you might accomplish more."
That's the final quote of the article. It's certainly true of agile development. We break production into manageable chunks and then we execute on them. If you find that you're jumping around too often, stop. It's not worth it. Perhaps it's time to engage in some Root-Cause Analysis to figure out what your motivation is for this disruptive habit.
I'll leave it at this. Here's an interview with the guy responsible for the Standford study.