Everyone has seemingly inexplicable sexual-energy slumps. You know, those times when bed simply means sleep, and an erotic interlude seems as tempting as a Gillian McKeith smoothie. But scientists have discovered that sexual arousal and levels of libido can be either dulled or stimulated by a whole host of external factors – from what we eat and how much we exercise to the state of our social diary. “What is going on in your life elsewhere will have a huge effect on your sexual response,” says the psychologist Petra Boynton, a sex and relationships expert. “Often, it doesn’t occur to people that they can improve their sex lives by making changes to their lifestyles, but they can. There are no quick fixes and no instant aphrodisiacs, but the bottom line is, if you are unhappy with your sex life, you might start trying to fix the ‘life’ part before you concentrate on the sex itself.”

EXERCISE

We all hope for a body like Elle Macpherson’s after a few sessions on the cross-trainer, but the benefits of exercise go beyond firm thighs and a healthy heart. Professor Judith Gerber ran a study at the University of Vermont college of medicine, which found that women who exercised five times a week had more satisfying sex lives than those who exercised three times a week, and that those who took exercise three times a week were more satisfied than women who did less. “Exercise enhances your body image, sense of wellbeing and energy level, all of which nourish sexual function,” she says. As well as seeming to improve self-esteem – those who exercise tend to rate themselves as more sexually attractive – exercise seems to help women achieve orgasm more easily and keep their sex lives going longer.

A study at the Center for Marital and Sexual Studies in Florida found that regular exercisers reached orgasm more easily and more often, and a survey of keen swimmers by Harvard University discovered that the sex life of a swimmer in her forties was like that of a 20-year-old.

As well as boosting mood and energy, exercise seems to change your hormone profile, raising levels of testosterone in women. Plus, the kick it gives to blood supply can provide an unexpected bonus. In a study at the University of Texas in Austin, women were asked to watch a sexy film; half of them had cycled for 20 minutes beforehand. The sexual response of the cyclists, measured by blood flow to the vagina, was 169% higher.

DIET

We all know about supposed aphrodisiacs, but food less exotic thanoysters and rhino-horn soup can affect sexual desire. In her book The Orgasmic Diet, Marrena Lindberg says: “In general, female sexual dysfunction is treated as a psychological problem. However, in my experience, many cases are linked to diet and nutrition, and women’s bodies respond much more effectively to diet than counselling.”

Lindberg claims that a high dose of omega3 fish oils can intensify orgasms, as it can boost levels of dopamine, a neurotransmitter increasingly seen as vital in stimulating desire, in the brain. “Certain foods can increase dopamine levels,” says Beverly Whipple, co-author of The Science of Orgasm. “Sugary, starchy foods increase levels of serotonin, which improves mood but can inhibit orgasm.”

To keep your dopamine-serotonin balance healthy, Whipple suggests a diet consisting of 40% whole grains, fruit and vegetables, 30% protein and 30% healthy fats, such as olive oil. And steer clear of soy supplements: they can decrease normal sexual behaviour by as much as 70%. The culprit is thought to be isoflavones, oestrogen-like molecules that seem to reduce the sex drive.

STRESS

Deadlines, money, work, children – life is full of stress. And stress is bad for your sex life. “Stress dampens libido,” says Dr Marilyn Glenville, a nutritional therapist and women’s-health expert. “This may be because a higher level of the stress hormone cortisol interferes with the healthy function of reproductive hormones. Moreover, when you feel anxious and unsettled, sex is probably the last thing on your mind.”

Cortisol can block the availability of progesterone, the sex hormone that switches on desire, which may leave you lacking in lust. So reduce your stress levels by exercising, meditating or getting an early night. Paula Hall,a counsellor for Relate, says: “If you’re tired, your body’s priority is sleep and recuperation, not sexual gratification.” Sleep reduces cortisol levels markedly and increases those of the growth hormone DHEA, which improves energy and wellbeing. And don’t forget, sex is an excellent stress-buster, too.

SOCIALISING

Meeting friends for lunch or going for a night out with a mate may be relaxing, but it’s also good for your sex life: a healthy social life reduces stress, makes you happy and more confident and can even make you more interesting to your other half. In one study, Boynton found that unemployed women were much more likely to be unhappy with their sex lives than those with jobs. “Feeling isolated will impair your sex life, whereas outside friendships tend to make for happier relationships,” she says. “If you do things outside the relationship, you have more to talk about. A good network of friends can also help to protect you from stress.”

Studies show that women get much of their self-esteem through their connections with others. “People with social support have fewer cardiovascular and immune problems, and lower levels of cortisol,”says Professor Tasha Howe of Humboldt State University in California.

TELEVISION

For many couples, the bedroom is not so much a haven of sensuality as a place to catch up with late reruns of CSI: Miami, before falling into a deep and sexless sleep. However, a recent study in Italy showed that people who had a television in the bedroom had half as much sex as those who didn’t, and that certain programmes – violent films and reality TV – were passion killers for one-third of couples.

And it might be wise to turn off MTV before it turns you off: research at Leeds University found that women’s self-esteem was lowered by watching videos featuring young, hard-bodied singers and models. Just 10 minutes of bumping and grinding left women feeling despondent about their own bodies, which hinders sexual satisfaction. “If you cut down on your viewing, you spend more quality time together,” says Boynton. “Chatting, reading, listening to music, sharing stories about your day, cuddling – the choice is yours. And you might find you feel more positive towards each other, which, in turn, could lead to more sex.”

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