The three-or-four-hours rule for getting creative work done

you almost certainly can't consistently do the kind of work that demands serious mental focus for more than about three or four hours a day.

But it's also true that we're increasingly the kind of people who don't want to rest – who get antsy and anxious if we don't feel we're being productive. The usual result is that we push ourselves beyond the sane limits of daily activity, when doing less would have been more productive in the long run.

in some vague way, but specifically to ringfence three or four hours of undisturbed focus