Why is it that a man feels at his sexiest when he wakes up in the morning but a woman wants to go back to sleep? And why, late at night, when a woman is in the mood for love, is her man just lying there, snoring? It all comes down to hormones. Here's why our sex clocks don't always tick in time.
Even before a man wakes up, his testosterone levels are at their peak -between 25-50 per cent more than at any other time of the day. That's because the pituitary gland -which governs the production of the male sex hormone -has been switched on in the night and levels have been steadily rising until dawn.
Women also make testosterone -the main sex drive hormone -but produce a fraction of the amount and it rises by only a tiny bit overnight. It is also kept in balance by oestrogen and progesterone. Men need only a normal amount of testosterone to feel like having sex. The raised levels in the morning means most men will wake up two to three times a week with erections.
Slept well? A good kip is another reason a man may feel even more amorous in the morning. Studies have found that the longer and deeper a man has slept, the higher his testosterone levels. Research in the Journal of the American Medical Association shows getting more than five hours' sleep can raise male levels by an extra 15 per cent.
While a man's levels of s*x hormones are at their highest when he wakes up, a woman's are at their lowest. Male and female testosterone levels are at their highest at opposite ends of the day, so they are out of sync, says consultant gynaecologist Gabrielle Downey, of the Sandwell and West Birmingham Hospitals NHS Trust.
Downey says it takes more than hormones to get a woman in the mood, so men need to make an active effort."
Women's hormones rise and fall more over their monthly cycle than through the course of a day. At their height -midway through her cycle -a woman's levels of testosterone will be 30 times higher than at the start.
As both sexes get ready for the day, levels of the stress hormone cortisol rise to help them wake up -and dampen the effect of sex hormones. Cortisol, studies show, lowers the sex drive of both men and women.
As the day goes on, men steadily make testosterone. The hormone is needed to trigger muscle growth and sperm production. Levels will fall and rise every 90 minutes through the day as part of the body's in-built clock.
An attractive colleague wanders into the workplace. But it's his nervous system and not his hormones that kick into action.
The sight of someone attractive immediately releases feel-good brain neurotransmitters called endorphins -and triggers blood flow to a man's genitals. Sex hormones take longer to increase. However, when confronted with a sexy person, a man who already has higher testosterone levels is likely to be more flirtatious. Men with more testosterone are also more likely to be found attractive by women, according to researchers at Wayne State University in Michigan, US.
Women who catch a glimpse of a hunk at lunchtime, are less likely to be aroused than if a male colleague sees someone he fancies. Instead, studies have found that a woman's testosterone levels are more likely to be boosted by the anticipation of sex with her own partner.
In one study by the University of Texas, women in long-distance relationships gave five saliva samples. The tests were given two weeks before they saw their partners, the day before, before sex, the day after sex and three days after the pair were separated. The women's testosterone levels hit their peak the day before they were due to see their partners again.
As evening approaches, men's levels of testosterone start to fall while women's sex hormones gradually rise.
Studies have shown, that an after-work gym session can boost the libido of both genders. Research by scientists at the University of California, San Diego, found men who exercised had a boosted libido.
They also reported having 30 per cent more sex and 26 per cent more orgasms. A University of Texas study showed that women who had 20 minutes of cardio were more aroused by seeing an erotic film than those who had not exercised.
When it's time to unwind after a stressful day, even putting on music can affect sex hormone levels. A Japanese study by Nara University found that music boosted testosterone levels “significantly" in women but had the opposite effect in men.
Researchers believe the reason is that music may make women feel more relaxed, bonded and in turn more sexual, while in men it calms their aggression and in turn reduces their levels of testosterone.
If an important match is on TV, the results can also affect a man's testosterone. According to a study of saliva tests by the University of Utah, sports fans watching a World Cup game boosted hormone levels by about 20 per cent if their team won.
After a loss in a big game, there was a 20 per cent drop in levels.
By contrast, women are more likely to be affected by actually playing sport rather than watching it.
By now, a man's testosterone level is dropping to the day's lowest level, while a woman's is heading towards its highest point. But, sexual desire in women is not a simple story.
Downey says, the greatest factor influencing sex drive is body image. If a woman sees herself as unattractive, she is much less likely to want to have sex. Which is why, women with polycystic ovaries, who have increased levels of testosterone, don't have increased libido. They often see themselves as overweight and not attractive -and that feeling over rides their heightened sex hormones.
Even though men's testosterone levels are now at their lowest, they are still more likely to have sex in the evening as men's levels are still running higher than a woman's, says Downey. She adds that once they are making love, if a woman's testosterone levels are peaking because of where she is during her cycle -around the 13th day -her orgasms are more intense and they will be felt all over the body.
When the levels are lower, her climaxes are less intense and centred mainly around the sexual organs.