Many expert performers engage in «deliberate practice», which requires deep concentration on areas that are just out of your reach and is best undertaken alone. (slaw.ca)
They know that having the maximum impact means leaving time for deep concentration and uninterrupted pondering (and yes, even adequate rest). (inc.com)
You could even be holding your breath when concentrating or carrying out a task that requires deep concentration. (weightlosswithsarah.com)