From Reader’s Digest:
A scientific study, published in the Journal “Neuron“, has found that multi-tasking does not make you more efficient.
Subjects were instructed to identify different-colored images while also identifying a variety of sounds. Researchers then monitored brain activity with an MIRI. When both color and sound were introduced at the same time or within a half a second of each other the brain simply delayed responding to one until the other was performed.
The conclusion was that the brain has a built-in “bottleneck” that prevents interferences caused by dueling thought processes. Some multitasking can be learned with practice (such as playing an instrument while talking). But many overlapping tasks (such as emailing while talking on the phone) are not really possible.
Lesson: You are better off completing one task before moving on to another; otherwise you compromise the integrity of both tasks.