The three-or-four-hours rule (via) by Oliver Burkeman.
The real lesson – or one of them – is that it pays to use whatever freedom you do have over your schedule not to “maximise your time” or “optimise your day”, in some vague way, but specifically to ringfence three or four hours of undisturbed focus (ideally when your energy levels are highest).
The truly valuable skill here isn’t the capacity to push yourself harder, but to stop and recuperate despite the discomfort of knowing that work remains unfinished, emails unanswered, other people’s demands unfulfilled.