The cognitive cost of task-switching measured in lost focus time
Attention residue is the phenomenon where part of your attention remains stuck on the previous task even after you switch to something new. Research by Sophie Leroy shows that when you interrupt a task mid-flow, residue from that incomplete task occupies working memory, reducing the cognitive capacity you bring to the new task. You are not fully present in the new task until the residue clears.
The residue costs real productivity. If you switch from email to deep work, your brain is still partially processing the email. You start writing code or analyzing data, but 5-10 percent of your processing power is still drafting replies or worrying about a message you just read. You type slower, make more errors, and need longer to enter a state of full concentration.
Minimizing residue through batching and clear demarcation
The best mitigation is to batch similar tasks together so you switch less often. Do all email in one block, all meetings in another, all deep work in a third. This reduces residue accumulation because you are switching between fewer contexts.
A secondary strategy is to close out the previous task mentally before switching. Write a one-sentence summary of what you accomplished and what comes next, or set a timer for finishing the task later. This signals to your brain that the task is captured and can be released, clearing some residue. Without this closure, your brain keeps the task active, waiting for resolution.