Researchers tested stress levels after public speaking

New Scientist magazine reports that Stuart Brody, a psychologist at the University of Paisley, found having sex can help keep stress at bay.

However, only penetrative intercourse did the trick - other forms of sex had no impact on stress levels at all.

Professor Brody monitored how various forms of sex affected blood pressure levels in a stressful situation.

For a fortnight, 24 women and 22 men kept diaries of how often they engaged in various forms of sex.

Then they underwent a stress test involving public speaking and performing mental arithmetic out loud.

Volunteers who had had penetrative intercourse were found to be the least stressed, and their blood pressure returned to normal faster than those who had engaged in other forms of sexual activity such as masturbation.

Those who abstained from any form of sexual activity at all had the highest blood pressure response to stress.

Dr Brody found that the effect remained even after taking differences in personality and other health-related factors into account.

Nerves stimulated

He told the BBC News website it was possible the calming effect was linked to the stimulation of a wide variety of nerves which takes place during heterosexual intercourse, but not other forms of sex.

You are probably better off thinking about what you are going to say and preparing thoroughly

Dr Peter Bull

In particular, the vagal nerve plays a role in controlling some psychological processes.

In addition, the release of the hormone oxytocin during sex might have a calming effect.

Professor Brody said it made sense in evolutionary terms for standard heterosexual sexual intercourse to be associated with a wide range of positive effects on behaviour.

He said: "A growing body of research shows that it is specifically intercourse, and not other sexual behaviours, whether alone or with a partner, that is associated with a broad range of psychological and physiological benefits.

"And greater frequency of intercourse is associated with greater benefits."

But Dr Peter Bull, a social and political psychologist at the University of York, said there were other ways to prepare for a speech that were more likely to reduce stress.

He said: "You are probably better off thinking about what you are going to say, and preparing thoroughly, rather than having sex the previous night."