How the world divides is a popular old parlour game, but in the hands of novelists it assumes a grander design. In an Independent interview on Friday, Martin Amis said that Zadie Smith divided between the organised and disorganised, Nabokov chose those who sleep well and those who do not, while Martin was at one with his late father Kingsley in separating the attractive from the unattractive. He did not mention Robert Benchley's witticism that there are two kinds of people in the world, those who divide the world into two kinds of people and those who don't.
I favour Nabokov's split as the most profound. You can teach yourself to become organised, and attractiveness does not inform character in the way that sleep does.
On the same day as Amis's pronouncement, the political biographer Bruce "The Brute" Anderson, observed that while Gordon Brown thrashes about at night as sleepless as the Ancient Mariner, David Cameron sleeps like a baby the moment his head hits the pillow.
This goes to the essence of the men. It is a battle between the sleepless and the sleep filled. The derided election poster of Cameron did not look particularly airbrushed, but it did show a man completely rested. This leads to the calm and self-contained quality noted by Jonathan Yeo, the portrait painter. "Cameron was happy in his own skin," he said last week. As opposed to Gordon Brown who has the skin of a vampire and Audenesque creases round his eyes.
This is not necessarily a bad thing. Brown sells himself on adversity. He is a leader for troubled times. He might encourage comparisons with a fellow insomniac, Winston Churchill. If we are about to enter a double dip recession, do we want a prime minister glowing with a Davos tan, or would we prefer someone who looked like a bag of old socks? If something is trivial, we say that it is "not worth losing sleep over". The corollary is that important matters are worth keeping awake for.
As Leonard Cohen said: "The last refuge of the insomniac is a sense of superiority to the sleeping world." George Bush's critics never forgave him for tucking himself up in bed by 10pm every night. You should not be able to sleep through a war if you are commander-in-chief, although soldiers are trained to sleep anywhere, even while standing.
Ideally, you need to look weathered without looking knackered. Jack Bauer gets it right in the television series 24. Most of us would start to nod off after about 18 hours, but Bauer manages without sleep, food or water. Some cup-a-soup and a snooze would destroy the tension and the heroism.
I think that Brown's private temper-related tiredness is permissible. Like Carlo Ancelotti, I judge our stars by their behaviour on the field. The Prime Minister may be clumsy and gaffe-ridden but nobody can doubt his stamina. David Cameron's supporters would say that he is just as dogged, but does not show the same wear and tear. In James Hanning and Francis Elliott's biography of Cameron, the dissection of the Hutton report coincides with the extreme suffering and multiple fits of Cameron's late son, Ivan. Yet Cameron never faltered.
I wonder if there is a sub-division of Nabokov's dictum. It is not those who sleep well and those who don't, but those who cope without sleep and those who don't. That gives Gordon Brown the chance of two new bedfellows, Margaret Thatcher and Napoleon.
Sarah Sands is deputy editor of the London Evening Standard