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You’ve got a presentation at 2 p.m. But there are also a few emails you have to get to before the end of the week, and you’ve also got to really start prepping for a Monday meeting. But it’s almost lunch and, well, you can eat at your desk, read up on whatever new thing Donald Trump said, and still bust out a few memos, right?

Wrong, argues Georgetown University computer-science professor Cal Newport’s new book, Deep Work: Rules for Focused Success in a Distracted World. In the book, excerpted recently on Wharton’s website, Newport attempts to understand how workers can rise above their infomania. His trick: deep work to conquer attention residue.

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. As Leroy’s abstract details:

[P]eople need to stop thinking about one task in order to fully transition their attention and perform well on another. Yet, results indicate it is difficult for people to transition their attention away from an unfinished task and their subsequent task performance suffers.

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.

Here’s where what Newport calls “deep work” comes in: He suggests focusing on a single, intense task for a long period of time to reach peak productivity. You don’t get attention-residue issues, which means your output is stronger, cleaner, and just plain better from a lack of distractions. So if you’ve got ten emails to write, block off some time to just focus on those emails. If you’ve got a presentation due tomorrow, put your away message on, sequester yourself, and focus on banging out that presentation. Most important: Don’t stop — or begin something else — until you’re completely done.