Multitasking between two demanding activities is not a separate skill that some people possess. It is rapid alternation between tasks, with a switching cost in both directions.

Lesson 3 explained why. The expensive part is not leaving a task. It is returning and reconstructing what you were holding in mind.

When you alternate between two demanding tasks, you pay that cost repeatedly.

There is an important distinction. You may be able to combine a demanding task with a routine one. For example, you might think through a problem while walking or tidy something while listening to a call in which you do not need to participate actively.

This is less costly because only one activity requires a high level of attention. The four states from Lesson 2 help explain the difference. Two focused tasks compete for the same mental resources. A focused task and a routine task may compete much less.

The rule is therefore more specific and useful than do not multitask: avoid doing two demanding things at the same time. Other combinations can be judged according to what works for you.

Here are four practical ways to reduce unnecessary switching:

• Use one active window, or as few as your work allows. Frequent switching between windows is associated more strongly with boredom than with focused attention. A large number of open tabs may not reflect demanding work. It may show that you are not fully engaged with any one task.
• Place your phone somewhere that requires a decision to reach. Do not leave it face down beside you. Put it in a drawer or another room. The extra effort interrupts the automatic habit of checking.
• Group similar tasks together. Answering six messages in one session requires you to enter that type of work once. Answering the same six messages at different points throughout the afternoon requires you to rebuild your previous task each time.
• Give the work session a clear end. Attention is easier to maintain during a defined period than during an open-ended task. Forty minutes with a stopping point gives the session a clear shape. Work on this until it is finished does not.

Do not set four uninterrupted hours of deep work as your standard. Few people can maintain that level of focus, and many jobs do not allow it.

An unrealistic standard can make every ordinary working day feel like a failure. Two protected periods of focused work within a day that still contains meetings and interruptions is a more realistic target and a meaningful improvement.