As David Davis gaily tripped out of Annabel's early on Wednesday morning, his grin said one thing: "It's Christmas!" With a brunette on one arm and a blonde on the other, the shadow Home Secretary exhibited behaviour uniquely associated with the party season. Shoes shined, tie loosened by a reckless half-inch, he was the classic straight-from-the-office reveller. One photo shows him gesturing garrulously at the camera, his mouth half open and his eyes half closed. He looks as if he is about to start crooning Frank Sinatra into the ear of his companion. In another, a gaggle of false friends cheers him on as a fur-clad bottom lowers itself on to his lap. A source from inside the nightclub reported: "I wouldn't say he was a good dancer, but he was certainly enthusiastic." The Christmas party season had been launched.

Unfortunately, the morning after the Christmas party dawns for failed Tory leadership contenders as surely as it does for the rest of us. The day after he wowed his co-workers with his devil-may-care footwork and his accommodating thighs, Mr Davis slunk back to his wife. "I am perfectly aware of [the photographs]," she told a reporter. "I have nothing further to say."

The pattern will be repeated in nightclubs all over Britain, as the Christmas party season staggers inexorably on. Mr Davis was not the first. That honour goes to two co-workers in Birmingham, rescued at 4am in the middle of November after climbing into the Botanical Gardens in a state of dishabille. Nor the worst. That could be a Mr Laidlaw in Cardiff, who chewed off a colleague's ear at a Christmas party when the man put his hand up Mr Laidlaw's kilt and "squeezed hard".

Preparing for this Yuletide high-spiritedness has become essential. Last week, Canon announced it has made the glass in its photocopiers 1mm thicker to cope with Christmas high jinks. Repairs go up by 25 per cent in the new year "due to rear-end copying", the firm said. "I repaired a machine with a photocopy of a man's groin jammed in it," reported one chirpy technician. "The manager suggested an office identity parade to see who Canon could charge for the call-out charge."

The revelry is almost part of the British constitution: after working hard all year, it is our inalienable right to completely and spectacularly humiliate ourselves for one night in December. Christmas just isn't Christmas if we haven't jabbed insults at the boss with a meatball on a stick, or succumbed to the stalkerish postboy in a Wham!-induced stupor of sentimentality. A survey of 2,000 workers by the dating website cupidbay.com claims that 38 per cent of workers have had sex at the office Christmas party; 35 per cent of those encounters were in toilets.

All this misbehaviour costs: the Chamber of Commerce says London companies spend £30 to £50 per employee on the annual knees-up. Partygoers, in return, will spend £18.5m on taxis home. Women will apparently spend two years of their lives getting ready for nights out (and men seven months, although in the case of Shane in accounts it is time wasted). "Overindulgence sick leave" will cost industry £65m over Christmas.

One survey claimed 80 per cent of employers may abandon the festivities for fear of sexual harassment claims. In October, the Government introduced legislation that makes it easier to define harassment. It has to do with "violating a person's dignity", and it is difficult to enforce a fancy dress code and obligatory karaoke without doing that.

Now, there is no suggestion Mr Davis's comely companions were trying to harass him sexually, but a piece of research may be of interest to his wife. A professor of clinical optometry at the University of Manchester has come up with an equation for "beer goggles", which makes hairy dullards appear strangely attractive as the drink flows.

Factors include the amount of alcohol consumed, the luminance of the object of affection in candelas per square metre and something called Snellen visual activity. Perhaps when Mrs Davis demands an explanation, the professor would kindly step in with his graph.