Tanya Basu, for The Science of Us:

Which means ... what, exactly? Newport explains it using a 2009 paper titled “Why Is It So Hard to Do My Work?” from Sophie Leroy, a business-school professor at the University of Minnesota. She studied a modern, daily workplace conundrum: switching between tasks and getting things done. In two experiments, Leroy finds that people are less productive when they are constantly moving from one task to another instead of focusing on one thing at a time.

...

Leroy calls this carryover from one task to another “attention residue,” where you’re still thinking of a previous task as you start another one. Even if you finish your task completely, you still have some attention residue swirling around your head as you embark on your next task, meaning that bullet point on your to-do list doesn’t start off on the right foot. In other words, as much as multitasking gets nods for being an asset in today’s time-crunched world, it’s not really a good thing when it comes to your productivity, and it's actually a time-waster.