The title Prime Minister has no job description, no set hours, and no way of monitoring how effectively time is spent. Whether or not prime ministers work too hard, then, depends entirely on their character and how they see their mission.
For the leader of a party that practises total politics, the temptation must be to overdo it.
Margaret Thatcher set a bad example, of course, being perhaps the first total Prime Minister, notoriously averse to taking holidays or days off.
Tony Blair followed her in that as in other respects. When his son Leo was born three years ago, the first to a serving prime minister for 150 years, Blair did not really take time off, cutting against the modern fashion for parental leave - not to mention his own government's family-centred rhetoric.
Of course, ambitious politicians do not get to the top of their profession by being lazy, but previous incumbents have tended towards the lifestyle of the leisured upper classes.
Harold Macmillan, for example, whose career as Prime Minister was cut short by just the kind of health scare that Blair has suffered, used to take two-week-long sojourns in Switzerland during which his only substantial work would be to draft a single speech.
The model of the premiership was then much more that of chairman rather than chief executive. The business of what is now known as delivery was delegated to the baronial departments of state. But Blair more than any of his predecessors, possibly even than Thatcher, becomes involved in direct supervision of managers delivering public service targets.
His character has always contained the element, in any case, of hyperactivity. He is surprisingly energetic for someone of such relatively calm public demeanour. Even before he became a politician, friends commented rather wearily on his enthusiasms, tending towards obsessions, and his habit of throwing himself at projects.
As the youngest Prime Minister since Lord Liverpool, Blair was able to do more than most of his predecessors, able to stay focused during long overnight negotiations in Northern Ireland or over European Union budgets (and on one gruelling occasion, both at the same time).
Before he assumed office, he had determined to maintain his fitness, which he has done with the same sense of purpose he applies to everything else. He was always conscious that his father suffered a serious stroke at the age of 40 - indeed, it was the spur to his own political ambitions - and knew that it was because of overwork.
Yet his response to that challenge was to work out, rather than to choose an easier path in life.
John Rentoul is author of the biography 'Tony Blair: Prime Minister', published by Time WarnerReuse content