Shifting mental gears takes time and energy. This is one of the reasons multitasking may seem more efficient on the surface, but may in reality actually take more time in the end.
The American Psychological Association (APA), in Washington, DC, is the largest scientific and professional organization representing psychology in the United States and is the world's largest association of psychologists. According to the APA article "Switching Costs", in experiments published in 2001, Joshua Rubinstein, PhD, Jeffrey Evans, PhD, and David Meyer, PhD, conducted four experiments in which young adults switched between different tasks, such as solving math problems or classifying geometric objects. For all tasks, the participants lost time when they had to switch from one task to another. As tasks got more complex, participants lost more time. As a result, people took significantly longer to switch between more complex tasks. Time costs were also greater when the participants switched to tasks that were relatively unfamiliar. They got up to speed faster when they switched to tasks they knew better.
In a 2003 paper, Nick Yeung, Ph.D, and Monsell quantitatively modeled the complex and sometimes surprising experimental interactions between relative task dominance and task switching. The results revealed just some of the complexities involved in understanding the cognitive load imposed by real-life multi-tasking, when in addition to reconfiguring control settings for a new task, there is often the need to remember where you got to in the task to which you are returning and to decide which task to change to, when.
Even more interestingly, Meyer has said that even brief mental blocks created by shifting between tasks can cost as much as 40 percent of someone's productive time.