One survey, published in the Journal of the American Medical Association, found that one in seven men report low libido, and for women, the figure is even higher - half of British women over 50 said they'd experienced a drop in their sex drive over the last five years. So what can be done to revive flagging interest? Here are our top 10 tips to get you going again.
The best way to make something more attractive? Put it just out of reach. Sex therapists often suggest that couples take sex off the agenda in order to resolve any problems they have. "It seems counterintuitive," says Dr Pam Spurr, author of Sensational Sex (Robson, £7.99), "but taking a break takes the pressure off, and encourages non-sexual intimacy, which in turn makes you both feel more easily aroused because you're relaxed."
Just two handfuls a day will pump up a man's sex hormones. Research has shown that men who have a diet high in mono-unsaturated fats - found in walnuts, peanuts and almonds - have higher testosterone levels than those who don't.
Let it flow
To get aroused you need good circulation - that's partly how Viagra works, by relaxing the pelvic area to enhance blood flow. Exercise will help keep your blood pumping and if you lose a bit of weight while you're at it, that will also help: a study at Duke University in North Carolina, USA, showed that slimming down makes both men and women more comfortable and enthusiastic in bed.
And make it cereal, which is a good source of zinc. "A deficiency of zinc can impact on your body's production of testosterone, making you feel unenthusiastic," says Dr Sara Brewer, author of Better Sex (Marshall, £7.99).
Keep it on
Building sexual tension gives you a far bigger release when you finally climax, "So rather than peeling off all your clothes before having sex in the bedroom, try some heavy petting with your clothes on - in the kitchen, the hallway or even the garden shed," says psychosexual therapist Vicki Ford. "It might feel daft at first, when you could just go to the bedroom, but changing the location of where you get aroused can trigger new desires and emotions."
Easier said than done, but depression is the cause of 30 per cent of low-libido cases, so do all you can to put a smile on your face. Eat bananas or Brazil nuts to boost your serotonin levels, essential for good mood; watch a comedy - even seeing smiling faces can lift your spirits; and get a dose of sunlight by taking a daily 15-minute walk outside. During the darker months, your body may struggle to produce enough vitamin D, which it usually gets via the sun's rays.
Watch an action film
Sixty men were asked to watch excerpts from The Godfather: Part II or a documentary about the Amazon - those who watched Al Pacino do his scary thing had higher testosterone levels than those who watched the wildlife documentary.
You may be wishing for less stress in your life, but being too relaxed could cause your libido to flag. "Real competition can drive up testosterone, which boosts libido," says Dr Helen Fisher, author of Why We Love (Owl Books, £7.99). "Being amped up by a high-powered, high-stress job is more likely to make you more sexually active than idling in a cushy, low-key career."
Testosterone is necessary for a healthy libido in both men and women, but you don't need to rely on work to provide competition; just watching a game of sport can increase a man's testosterone levels by up to 15-20 per cent and a woman's by 24 per cent, according to a Pennsylvania State University study.
Eating chocolate could help encourage arousal - the dark stuff contains methylxanthines, which are stimulants that increase your skin's sensitivity to touch. Cup of cocoa before bedtime, anyone?
Playing with yourself makes you more likely to want to play with your partner. "It's a surprising correlation," says Fisher. "But it's probably because sexual arousal elevates testosterone and dopamine, and that can lead to more sex."