When your relationship has gone beyond the first flush of passion, maintaining your libido can be a delicate balance of health and desire.
And it doesn't help that numerous things - from everyday foods to gadgets - may interfere with your sexual performance.
Here, Good Health looks at some of the more surprising reasons your love life could be off-kilter . . .
1. A POOR SENSE OF SMELL
Smell plays an important role in our recognition of pheromones - the scents given off by a partner's body through sweat - and affects sexual behaviour.
A 2012 study at the University of Gothenburg in Sweden found that men with no sense of smell had significantly fewer sexual partners than those with a normal nasal function, and women with no sense of smell felt less secure in their relationships.
Psychologist Dr Ilona Croy, who led the study, says having impaired smell will give you a disadvantage in the mating game.
"A lot of social signals are transported through the olfactory channel (the nose lining and associated nerves) and a depleted sense of smell makes it likely that you might miss them," she says.
Simply having a cold could also affect the sex drive of both men and women, adds Dr David Edwards, a GP and sexual health specialist.
"Viral infections can temporarily reduce testosterone levels," he explains.
2. GOING ON A DIET
If you're trying to slim down, it could have an unwanted side-effect.
Losing weight too quickly puts stress on the body - even if you are a healthy weight - which can trigger the release of a hormone called prolactin, a powerful suppressor of sex drive.
"Stress like this can also mean a reduction in oestrogen and testosterone levels, which is why a woman's menstrual cycle can become irregular," says Dr Sarah Brewer, a GP and the author of Overcoming Low Sex Drive.
Losing more than 10 per cent of your weight too quickly can make the body think it's starving, which can cause sexual interest to fall, she adds.
3. SLIPPING ON A PAIR OF HEELS
THEY may look sexy, but high heels could make sex less enjoyable for some women.
Dr Eden Fromberg, a New York-based gynaecologist, says that because heeled shoes tip the body forwards, this forces the postural muscles to contract as the body works to maintain an upright posture.
"If the pelvic floor muscles are chronically contracted, they can become tight and therefore restrict the passage of nerve impulses going through them to the reproductive organs and arousal tissue," she says.
"Wearing high heels all day can affect a woman's orgasm - there is a reported improvement in female patients who retire their heels or use them only rarely."
4. HAVING A DESK JOB
A sedentary job is just as bad for a woman's potential to orgasm, warns Dr Fromberg.
"If you sit with rounded shoulders and your tail tucked under for hours, the muscles of your pelvic floor and the psoas muscle (which runs diagonally from the lower back through to the groin) remain chronically at 'half mast' - not stretched or contracted," she says.
This can affect the signals passing along important nerves leading to the reproductive organs. "Bucket chairs and recliners are the worst for exacerbating the sorts of postural problems that can lead to compromised sexual pleasure," she adds.
The body is designed to move constantly, so the more active you are, the more effectively blood supply and nerve signalling can work.
5. BEING HOOKED ON REALITY TV
People who have a TV in their bedroom have sex half as often as those who don't, according to a 2006 study of 523 Italian couples.
The effect is more marked for the over-50s, with the average of seven couplings a month falling to just 1.5 on average.
The study found certain programmes - violent films and reality shows - impede passion.
According to Dr Edwards, good sexual relations depend on couples communicating with each other, not sitting side by side watching television.
"If whatever you are watching makes you sad or grumpy, it is more likely to affect your libido," he says.
Technology can affect your love life in other ways, too. Sitting and spending more than seven hours a day on an electronic gadget (such as a phone or tablet) can trigger back pain, according to the British Chiropractic Association, and a recent survey of 2,000 people by backpainhelp.com found that 25 per cent had avoided sex because of back pain.
Men who carry a mobile phone switched on for at least four hours a day are at greater risk of erectile dysfunction than men who use phones intermittently, reported a study last year in the Central European Journal of Urology. One suggestion is that heat could be to blame.
6. YOUR "CUDDLY" LOVE HANDLES
As well as triggering energy slumps, excess sugar in the diet is laid down as fat, which can raise oestrogen levels, reducing the effect of testosterone in women and men.
"The more body fat you have, the more chance the testosterone in your system will be bound into the fat, which means less 'free testosterone'," says Dr Edwards. "Not only will this reduce libido, it makes you more likely to gather fat in your abdomen, which binds to any remaining testosterone, so diminishing your available supply."
Fat around the middle is a particular problem, because it can lead to a reduction in a sex hormone-binding protein called globulin, which is produced by your liver, explains Dr Mark Vanderpump, a consultant endocrinologist at the Royal Free Hospital, London.
This protein attaches itself to testosterone, transporting it around the body. "Men with lower levels tend to have a lower concentration of testosterone," he says.
7. THAT WEEKEND BIKE RIDE
Men who exercise regularly have a 70 per cent reduced risk of erectile dysfunction compared with men who do none - but serious cyclists are more likely to suffer impotence.
It seems the firm, angular saddles can put pressure on nerves and arteries around the groin, reducing blood flow to the penis.
"If there is bruising to the pudendal nerve (a major nerve in the groin that runs from the genitals to the base of the spine), erectile dysfunction can last for up to two years," says Dr Edwards.
But it's more typically seen on Mondays or Tuesdays after a weekend in the saddle, he adds.
He recommends saddles with a deep groove down the middle - to relieve pressure on the nerve - or putting a cushioned gel pad on top. "If cycling leaves you with a numbness or tingling in your groin, it's time to adjust your seating arrangements."
8. YOUR FEET ARE JUST TOO COLD
Women need warm feet for orgasm, suggested a Dutch study published in 2005.
Researchers revealed that 80 per cent of women were able to achieve orgasm when they wore socks, compared with 50 per cent when barefoot.
According to Professor Gert Holstege, a neuroscientist at the University of Groningen who led the study: "The feet play a significant role in maintaining body temperature, and by regulating internal climate the mind and body can come into a relaxed state needed for an orgasmic release."