So just who was it for you last night?
Nine out of 10 people think of someone else when they are having sex. That is a pretty startling, and even unsettling, figure.
But it's probably not so surprising to discover, as condom maker Durex did, that the most likely man for British women to fantasise about is Daniel Craig.
Strangely, George Clooney only made it to number four on the list, beaten by Clive Owen and Jude Law. But then, the British are nothing if not loyal to their own.
Not quite so loyal to their actual partners when it comes to fantasies, though, according to a book Sex & the Psyche by psychotherapist Brett Kahr, who surveyed 19,000 adults in the UK.
In research reminiscent of the ground-breaking US feminist tome of the seventies My Secret Garden by Nancy Friday, Kahr delves a little deeper, trying to discover why exactly we need fantasies.
He believes our secret fantasy life is why so many couples make love with the lights off: "It's nothing to do with hiding our flabbiness, but because a darkened room allows the secret cinema of our minds to unfold in a full, uninterrupted manner.,While we fantasise at liberty about celebrities, about one third of us also think of work colleagues, partners of friends, or close friends, when we have sex.
One woman in the book, Naomi, says she has dreams of Phil Mitchell from EastEnders making love to her on the carousel at Heathrow Airport -- definitely a case of a woman with baggage.
An Irish woman would be hard-pressed to envisage the same scenario in Dublin Airport, without being haunted by visions of angry holiday-makers trying to clamber over you to see if they were at the Lanzarote or Majorca belt.
But, apart from being a few minutes of harmless fun, Kahr actually believes that fantasising about others has a very important role.
Its power helps us balance our lives, which may explain why some sexual dreams are actually quite violent.
In life, we are driven by the dual urges of sex and aggression. Most of us suppress feelings of violence towards people and an aggressive fantasy can help us let off some of that steam. It can also help us to deal with distant trauma.
In the book, Hannah, now 48, had a hurried and rough first sexual experience. In her fantasy, however, she loses her virginity to Donny Osmond -- in her mind the epitome of a kind and gentle husband.
In her 'dream' she cries and he pats her face with a handkerchief and stays with her afterwards, in complete contrast with her own past.
Another entry in the book, Jenny, watches her hero Jimmy White win a snooker championship after she has made love to him during the interval. Jenny suffers from low self-esteem, so is able to counteract this with her dream of exercising influence over her imaginary lover.
But do Irish women have the same kind of fantasies as their UK counterparts, or has our new-found confidence meant it is largely irrelevant to us?
Sheila, a 37-year-old marketing executive, says she used to let her mind wander to the likes of Clooney and Brad Pitt during sex, until she met 'The One'.
"In a way, that was how I knew I had met Mr Right," she says. "I no longer needed to replace the man in my bed with one in my head.
"I couldn't think of anyone else I would rather be with, and I didn't want to spoil the intimacy by bringing someone else into the equation.,Karen, 39, who works in advertising, also admits that her current man is her 'McDreamy' and she has no need for a fantasy image, so I asked her about the past. "I was always so happy to have any man in front of me that I didn't have a fantasy man then, either!" she laughs.
Of course, when you offer the word 'fantasy' to most Irish men, the instant reply will be 'threesome'. Richard, a 28-year-old accountant, says he finds it hard to get images of Beyonce Knowles out of his head when he is with his wife.
"I don't want to exclude my wife, but I have these wonderful visions of both of them together, pleasuring me, for hours at a time," he admits cheerfully.
Another Irish male, a separated farmer, has a secret fantasy involving his Polish cleaners. When he is making love to his current girlfriend, he replays a scenario of him coming home from work early to find both women in his double bed and he joins them.
But note the importance of drawing the line, warns Kahr. If we take our fantasies too literally we can interpret them, mistakenly, as a command from our subconscious. We should rarely act on them, he says.
While they might be giving us subliminal messages -- e.g. we sometimes superimpose another person because we feel trapped by the one with are with -- sexual imagery is never usually that simple.
And sometimes acting out your fantasy can be a huge anti-climax. As a teenager, I regularly dreamt of passionate nights of lust with 1980s singer Midge Ure.
But the bubble of my little fantasy was well and truly burst some years later when he walked past me after getting off the stage in Whelan's of Wexford Street and I realised that even in my flat sneakers, the fantasy man of my mis-spent adolescence was a good deal smaller than me.
I felt more like patting him on the head as he shuffled past me, rather than having my wicked way with him in the dressing room!
These days I am more likely to have a woosy moment over thoughts of bad boy Roy Keane or Tyrone's Owen Mulligan, happy in the knowledge they both tick my 'height' box at least!
Gay writer Will, 32, responds within seconds to my text query: "Boris Becker, sweaty shorts, barley water and grunting noises.,And his straight friend Mick adds: "Madonna with a whip and a cross, I imagine myself assisting her in a remake of The Exorcist crossed with her Like a Prayer video.,But we also need to be careful of sharing our fantasies with others too.
Writing on the website healthyplace.com, sexologist Robert Birch warns: "Whereas honesty is usually the best policy, discretion must be used in the sharing of some unusual fantasy or fantasies involving other people. It is rare that a couple can share such private thoughts without, at best, a little discomfort. Too often the reaction to hearing a partner's most kinky fantasy is one of jealousy and distrust, if not anger and disgust."
"Most fantasies are just private thoughts," he adds, "that need not have a complex storyline, or a cast of hundreds. Working too hard at building a sexual fantasy can become a distraction, defeating one of its purposes. The best fantasies are often quite simple and tied in with pleasant memories.,But remember: if you start to fantasise about other people, it doesn't necessarily mean that your relationship is in crisis, says Kahr. It might just be a sign that some aspect of the relationship needs attention. Among the checks to be made are mid-life crises, bereavement, depression, poor career, drink, or even a new baby in the home. In these instances it may be worth contacting your GP or a therapist for further advice.
In general, though, it's certainly worth remembering that the reason fantasies are so cherished is because the majority of them will never, and should never, be realised.
Sex and the Psyche: The Untold Story of Our Most Secret Fantasies Taken from the 'Largest Ever Survey of Its Kind' by Brett Kahr, published by Allen Lane / Penguin Press, €37.25