IF ALL you want for Christmas is a good night's sleep, try sex. Doctors have admitted that making love last thing at night does have a sedative effect - with none of the 'hangover' problems of sleeping tablets. There also appears to be truth in the rumour that men tend to 'zonk out' after lovemaking, leaving female partners wide awake, ready for a cuddle and a chat.

The subject of sex was debated at length by sleep specialists preparing guidelines for GPs on the management of insomnia. Published this month by the Royal Society of Medicine, the guidelines tell doctors how to take an accurate sleep history and offer practical advice to patients, and when to prescribe hypnotic drugs. But sex, it appears, is a drug in itself.

'It is well known that vigorous exercise late at night is not conducive to sleep,' says George Beaumont, an experienced GP and sleep specialist who helped to compile the guidelines. 'Sports centres open later and later these days, so people are thrashing around on squash courts at 10 o'clock. They then wonder why they cannot get off to sleep.

'But sex doesn't quite fit the picture. Sex is a pretty violent kind of exercise, and when you think of the physical changes which the body undergoes during intercourse, you might imagine that these would stop people sleeping. Yet sex seems to induce sleep rather than interrupt it.'

The doctors admitted they were puzzled as to why sex and sleep go so well together. But Alan Riley, a specialist in sexual medicine and editor of the Journal of Sexual and Marital Therapy, says there is no comparison between a sweaty, competitive ball game and a satisfying sexual experience: 'Sexual arousal, and particularly orgasm, is associated with the release of opiates, which almost certainly contribute to the feeling of relief and tiredness after sex,' he says.

'Endorphins released into the brain during orgasm have a sedative quality and are pain-relieving, so this would help anyone who cannot sleep because of pain.

'Orgasm is also an amnesiac - it makes you forget things like the stresses of the day. Sex is physically relaxing because of the sudden release of muscle tension, and it is emotionally soothing, too. Some people masturbate just to get off to sleep.'

Nowhere in the new sleep guidelines for doctors does it suggest that sex will help you drift into dreamland. But the list of common causes of sleep disorders includes 'anxiety' and 'arousal', either of which could be brought on by lack of sexual satisfaction. Other causes include jet lag, shiftwork, napping during the day and rheumatic aches and pains.

'One of the most potent causes of not sleeping is anger,' says Dr Beaumont. 'And one of the things that people get very angry about is sexual relations. If there is a problem there, and your partner won't take part in sex and you are feeling strained, that is a recipe for insomnia. We have found that stress is the single biggest reason for people not sleeping well.

'I have a hunch that the sleep- inducing effect of sex depends on the orgasmic experience. Lack of satisfaction could be a powerful cause of staying awake.'

The new sleep guidelines were issued to coincide with the results of a survey investigating the incidence and effects of insomnia. Almost 2,500 adults aged between 18 and 64 were questioned about their sleep habits, and 21 per cent were found to suffer from either not being able to fall asleep, night- waking, or waking too early in the morning. That figure extrapolates to more than 5 million people in Britain who will have a problem sleeping tonight.

Insomnia can have devastating effects on the quality of life, leaving people moody, tense and ineffectual. When bad sleepers were asked about the problems they experienced after lack of sleep, they admitted to feeling irritable (54 per cent), being too tired to go out socially in the evening (27 per cent) and to having arguments or experiencing tensions with their partners (26 per cent).

So while an unsatisfactory sexual relationship may be one factor in a poor night's sleep, a poor night's sleep is likely to add to sexual tension. For some couples, sleep and sex can be enmeshed in a particularly vicious circle.

So it seems logical, if you wake at three in the morning and cannot get back to sleep, to reach over to your partner for a quick dose of endorphins. The survey, sponsored by the drug company Rhone-Poulenc Rorer, asked people exactly what they did get up to in the middle of the night. Sex was not on the list.

Apparently, most people try to ease their sleeplessness by reading, making a drink or watching television. Some even get on with the ironing, while others resort to sleeping tablets. But doctors advise against the use of hypnotics in the middle of the night. Caffeine, nicotine and alcohol should also be avoided close to bedtime.

While men in the survey said they were kept awake by anxieties to do with work, status, promotion or redundancy, women were far more likely to be worrying about relationships, the family and issues of love and support.

Which might begin to explain why - anecdotally at least - men are prone to roll over and fall asleep straight after sex, while women tend to want to continue the intimacy with pillow talk. Women I spoke to confirmed that their partners were quick to fall asleep after making love. 'He's out like a light,' said one. 'I wish I could go to sleep so easily.'

Paul Brown, a consultant clinical psychologist specialising in the treatment of sexual difficulties, believes any difference in post-coital experience has an evolutionary basis.

'The man's falling asleep is an extremely deep-seated reaction,' he says. 'If his primary evolutionary job is to impregnate the woman, he must do that as completely as possible.' And, having given his all to the sexual act, he simply goes to sleep.

Where does that leave a woman? Doctors agree it may depend on her orgasmic experience. Dr Brown even thinks it may historically be the woman's duty to sleep lightly and watch out for intruders. 'In evolutionary terms it was extremely important for one member of the partnership to stay on guard.'

In some primitive tribes, women not only feed their babies through the night, they tend the fire, too. But when a group of men leave the village on hunting trips, they also have to be able to stay alert at night.

Members of the Tauripan tribe, travelling through the South American jungle, wake in the dead of night to tell each other jokes. The hunters, even those deeply asleep and snoring, wake up to appreciate the gag, have a good laugh and resume sleep in seconds.

This blatantly relaxed behaviour contrasts sharply with the Western attitude to sleep. We put great stress on the standard eight- hours-a-night prescription. Yet surveys consistently show that fewer than a third of young to middle-aged adults wake in the morning feeling rested and refreshed. Insomnia is one of the commonest complaints taken to the family doctor.

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