“When you switch from Task A to Task B, your attention doesn’t immediately follow – a residue of your attention remains stuck thinking about the original task. People experiencing attention residue after switching tasks are likely to demonstrate poor performance on that next task.” So even if you don’t try to multi-task and focus on just one thing at a time, each time you switch to a new task you are getting sub-optimal performance on the new task.”

- From Deep Work by Cal Newport