It was 3am and she'd had a busy night. So it was only natural that the Duchess of York succumbed to the inevitable and nodded off in the back of a Mercedes taxi taking her home.
But she had a rude awakening – when her slumbers were interrupted by a passing photographer.
Sarah Ferguson was caught off guard in the early hours of Thursday as she dozed in the back of the luxury taxi. The flash of the camera woke her and, clearly disoriented, she ordered her driver to speed off.
An onlooker said: "It was a bizarre situation. It was 3am and the Duchess of York was asleep in the back of a taxi stopped at traffic lights on Cromwell Road in Kensington, West London. "When the flash went off she awoke with a look of horror on her face and told the driver to make off as quickly as possible.
"You certainly don't expect to see a member of the Royal Family snoozing in a cab, especially at 3am."
Sarah was not wearing a seat belt as she slept. As the car was in traffic, she was breaking the law.
And the photos have prompted speculation about what she was doing at that time of night.
The former wife of Prince Andrew was wearing a fashionable black leather blazer and lacy fishnet-style tights with a distinctive floral design on the calf.
The Duchess, an ambassador for WeightWatchers, could have been eating in one of the many restaurants in Kensington.
Or she may simply have been tired after a day's house-hunting.
She is said to be looking to buy a £1million apartment for her daughter, Princess Beatrice, who is expected to start a design course next year at Goldsmiths College in London.
The simplest explanation could be that her busy transatlantic lifestyle is taking a heavy toll.
The Duchess regularly commutes between Britain and America, where she runs a successful charity, writes children's books and works as a part-time television presenter on the Larry King show on CNN.
Last week, there was no sign of Geir Frantzen, her close friend and boss of the Findus food empire, who has been spotted in her company in recent weeks.
Last month the tycoon – who at 40 is eight years the Duchess's junior – escorted her around some of his favourite spots in Lofoten, a remote archipelago in northern Norway, where the pair were seen dining on whale burgers.
Friends say their relationship is strictly professional and deny reports she is planning to buy a holiday home in Norway.