Margaret Thatcher is famously said to have slept for only four hours a night. How easy is it to do a high-powered job on this amount of sleep?
Part of Margaret Thatcher's fearsome reputation came from how little she slept. She could get by on four hours a night, it has often been said.
Indefatigability became part of her mystique. She would keep her officials up working on a speech until two or three in the morning and then be up by five in time to listen to Farming Today.
"She slept four hours a night on weekdays," says Sir Bernard Ingham, her Downing Street press secretary. "I wasn't with her at weekends. I guess she got a bit more then."
It isn't easy to ascertain when Baroness Thatcher first referred to her minimal sleep schedule, but the figure of four hours has passed into lore.
People use it as a benchmark of endurance, often jokingly referring to those who need much more.
Lady Thatcher's close friend and former Conservative Party treasurer Lord McAlpine stayed with her at Chequers during the holidays. "She worked right through Christmas. When everyone else went off to bed she went off to work."
Her biographer John Campbell, author of The Iron Lady, says her late-to-bed, early-to-rise routine made her the "best informed person in the room". Occasionally husband Denis would snap. "Woman - bed!" he is reputed to have shouted on one occasion.
Her frugal sleep pattern created a problem for her successor John Major. "He found it difficult coming after her because the civil service had got used to a prime minister who never slept, and he used to sleep eight hours a night," Campbell says.
Sleep comes to be seen as part of a leader's character. When Napoleon Bonaparte was asked how many hours sleep people need, he is said to have replied: "Six for a man, seven for a woman, eight for a fool."
For the Iron Lady four hours was a badge of almost superhuman strength. It fits the narrative of the "warrior" prime minister as set out by the Times' Matthew Parris this week. "She understood that this was war when others didn't. And in war you need a warrior," he writes.
Churchill survived on four hours a night during the war. But what is less often noted is that he had regular afternoon naps in his pyjamas. Lady Thatcher was not one for these afternoon sleeps. "No, she wasn't a napper," Ingham says.
But is the four-hour measure something ordinary people should aspire to?
In the world of business it is certainly something people strive for. High-profile chief executives from Marissa Mayer at Yahoo! to Pepsi's Indra Nooyi get by on four hours a night, while Donald Trump claims to survive on three.
Geraint Anderson, author of City Boy, who worked as an analyst and stockbroker for 12 years, recognises the phenomenon.
"There was a real macho competition in the City about sleep. One of the ways of getting respect was bragging about how little you got."
The hours were long - from 6.30 in the morning to seven at night. Socialising might mean staying out till three in the morning. And this was just the analysts. The corporate financiers were the real hard workers.
"They'd work into the early hours, get a couple of hours' kip at the office and start again."
To admit needing sleep was a sign of weakness: "After the Christmas or summer party you'd make sure you stayed the latest and came in a little earlier than normal the next morning."
Margaret Thatcher was not the cause but her name was regularly invoked by his bosses. "They'd say she can get by on four hours to run the country. And she's an old lady."
As well as business, there have been military leaders who eschewed the eight hours and opted for the Spartan Thatcher credo. General David Petraeus ate one meal a day and slept only four hours a night, it was reported.
There's no correct amount of sleep, says Prof Kevin Morgan, of Loughborough University's sleep research centre. The only rule is to sleep long enough to feel refreshed when you wake up.
For about 1% of people - probably including Thatcher - this will be as little as four hours a night, says Morgan.
You can't just suddenly become someone who sleeps this little, he argues. It's likely to have been a pattern common to her life before becoming prime minister.
It is a big advantage for visionary or creative people to be part of this so-called sleep elite. And for a statesman attending all-night summits it might be a huge advantage.
"The people around you are flagging. When people get tired the quality of their decision-making is compromised."
Prof James Horne, also at Loughborough's sleep research centre, says that mood is critical. Soldiers high on adrenalin can function on little sleep: "It all depends if one gets a buzz out of what one's doing. If you're despondent, you tend to sleep more; if you're excited you need less. Margaret Thatcher was someone who felt on top of things."
The average adult sleeps seven hours a night but many sleep considerably less than this, especially people over 50. So it's possible that Thatcher fell within the range of normality rather than the 1%, Horne argues.
She may have sometimes slept four hours and made up for her deficit by sleeping a little longer on other nights; "You tend to attribute great things to great people, that they need no sleep or no food and have superhuman qualities."
Parris, who was a fellow Conservative MP of Thatcher's during the 1970s and 1980s, says it was probably more like four to five hours rather than the three to four that some have suggested. It took its toll and may have led to poor decisions, he believes.
Despite her toughness, she was often tired out, he remembers. "When we were jammed into the lobby I would be looking at her from six inches away. I would often see the eyes of an exhausted woman."
Recently there has been a move away from ostentatious sleeplessness. Burning the midnight oil in Gordon Brown's case was perceived as evidence of obsessive worrying and weakness.
The work-life balance has arrived, even in Number 10. Blair slept longer than Thatcher and Brown but made an exception to get up at night for baby Leo. George W Bush was in bed by 10, unlike his predecessor Bill Clinton, who worked late and got by on four or five hours.
For artists, sleep deprivation carries a whiff of creative drive and raucous hedonism. Keith Richards, the Rolling Stones guitarist, once stayed awake for nine days - when he fell asleep, he fell down so quickly that he broke his nose.
He makes a worse advert than Lady Thatcher did.