What is Cal Newport deep work context switching?

Example: You sit down at 9:00 AM to write the product roadmap — your ONE thing (The one thing question), placed in your peak window (Peak energy for peak work). Fourteen minutes in, a Slack notification appears. You glance at it: a teammate asking about an API change. You switch tabs, read the thread, type a three-sentence reply. Ninety seconds of clock time. You return to the roadmap. But you are not returning to the same cognitive state. Your brain is still holding fragments of the API question — the schema migration, the breaking changes, the teammate's deadline. Sophie Leroy calls this attention residue: the cognitive traces of Task A that persist into Task B. You reread the last paragraph you wrote, trying to reconstruct where you were headed. Three minutes pass before the thread of thought reconnects. Five minutes after that, your phone buzzes — a calendar reminder for a 10:00 AM check-in you forgot to reschedule. You start thinking about the agenda. The roadmap recedes again. By 10:00, you have spent fifty-one minutes at your desk and produced twenty-two minutes of actual roadmap thinking. Twenty-nine minutes — 57 percent of your peak window so far — have been consumed not by the interruptions themselves but by the switching costs surrounding them.

This concept is part of Energy Management in the How to Think curriculum.