Came across an interesting study (Quality Software Management: Systems Thinking by Gerald M. Weinberg) that draws relation between number of tasks in progress and time wasted for context switch.
So if you are running two projects (do two tasks) paralleled, waste caused by brain context-switch is 20% (and you’re able to use effectively only 80% of your time), for three it’s getting worse – 40% wasted (only 60% used), etc…
The mechanics behind are simple: according to psychiatric studies when our brain is juggling different tasks, it is also trying to arrange focus and attention to those tasks. Therefore when we attempt to perform two complex cognitive tasks, the brain must shift its focus to manage one task at a time. This process is identified as “reaction-time switching costs” and represents a measurable period of time in which the brain is moving its focus from one task to another pretty much as computer’s CPU does.
The other consequence is that we get tired earlier, since switch itself eats-up brain energy and we are not able to do 100% switch (we’re not CPU, we’re humans!), so it is more like having main part of brain focused on one task, while some small part of it still stays with next (past) one.
All this makes limiting WIP (work-in-progress) vitally important to both productiveness and human health.