The latest addition to the body of evidence came last month when Professor Stuart Brody of the University of Paisley published a study showing sex can lower blood pressure. "We're not just talking about the immediate effects of having had nice sex. The beneficial effects could last at least a week," says Professor Brody.

One theory is that intercourse stimulates a variety of nerves, most notably the "vagas" nerve, which is directly involved in soothing and calming. But you have to go the whole heterosexual hog. According to Professor Brody, studies show "penile-vaginal intercourse is the only sexual behaviour consistently associated with better psychological and physiological health". Such sex has been linked, in women, to a heightened emotional awareness, possibly because the "love hormone" oxytocin is released. One study even found that semen is a mood-enhancing ingredient. Doctors speculate that this is because semen contains several other mood-altering hormones — including testosterone, oestrogen, prolactin and several different prostaglandins — which can pass into the woman's bloodstream. This explanation, says Dr David Hicks, sexology specialist and consultant in GU medicine at the Royal Hallamshire Hospital in Sheffield, "is certainly feasible". Condom-free sex has its drawbacks, of course: contracting a sexually transmitted disease or becoming pregnant unintentionally.

If you are dogged by the sniffles at this time of year, regular love-ins could work wonders for your immunity — condoms and all. Psychologists have found that people who have sex once or twice a week have levels of immunoglobulin A (IgA) that are up to a third higher than their more restrained peers. IgA is an antibody that boosts the immune system and is the first line of defence against colds and flu. The health benefits for middle-aged men are also particularly persuasive. Recent studies suggest that men who have orgasms twice a week are half as likely to die early as men who orgasm less than once a month.

The more frequently men ejaculate, the less likely they are to develop prostate cancer, and if middle-aged men have sex twice a week or more they also have a lower risk of heart attack. Much has been made of the slimming and toning effects of a sexual work-out. In fact, sex probably burns off about the same number of calories per minute as a brisk walk. "You get all the benefits of exercise," confirms Dr Hicks. "This includes the release of endorphins, raised heart rate, moving the muscles and joints." You might also look younger. "Regular sex makes you feel younger as you are more relaxed, satisfied and less stressed," says Dr Kevan Wylie, consultant andrologist at the Royal Hallamshire Hospital. "Sex has huge mental benefits," adds Dr Hicks. "It's the outward proof that you are wanted, desired and valued." And if you feel sexy, you tend to look it, too.

Sex is not, sadly, a cure-all. "There is a danger in thinking that we can fix anything by leaping into bed," says Dr Petra Boynton, a psychologist at University College in London specialising in sex and relationships. "The fact is you are likely to have a much better sex life if you are healthy and happy, rather than the other way round." Still, it can't hurt to try. TELEGRAPH