By DANIEL MARTIN

Last updated at 13:12 22 February 2007

If you're facing a problem you can't get your head around, you might be told to sleep on it.

Now scientists have shown that the advice is more than just an old wives' tale.

Apparently sleep strengthens the memory and helps the brain extract themes and rules from the masses of information we soak up during the day.

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Lead researcher Bob Stickgold, professor of psychiatry at the Harvard Medical School, said: "We're not just stabilising memories during sleep. We're extracting the meaning.

"Sleep helps us extract rules from our experiences. It's like knowing the difference between dogs and cats, even if it's hard to explain."

The US research team studied how well participants remembered connections between words and symbols. They compared how they fared if they had had a sleep between seeing the words and having the test, and if they had not slept.

They found that people were better able to recall lists of related words after a night's sleep than after the same time spent awake during the day.

They also found it easier to recollect themes that the words had in common. But they forgot around one in four more themes if they had been awake.

In another experiment, people were shown cards bearing symbols, followed by descriptions of a particular weather condition. In the test, a diamond shape might be followed by rain for 70 per cent of the time.

Twelve hours after training, participants felt able to predict the weather based on the symbol shown to them on a card. But after they had slept, their predictions were 10 per cent better.

Prof Stickgold presented his findings, which were reported in Thursday's edition of New Scientist, at a meeting of the Science Network in California last week.

Other recent studies have demonstrated the benefits of sleep.

Last year it was shown that taking a daytime nap helped boost the memory and make it easier to recall important facts.

Another study showed that men's brainpower can be reduced if they share a bed, because their sleep is more disturbed.

Middle-aged people are at an increased risk of high blood pressure if they sleep for less than five hours a night - and afternoon naps cut the risk of heart disease.

Sleeping can also make you thin: women who only sleep for five hours a night are more likely to end up obese than those who sleep for seven hours.

But sleeping too much is not good for the health. People who sleep for nine hours a day are more likely to develop Parkinson's disease than those who sleep for six, another study showed last year.

John Groeger, professor of cognitive psychology at Surrey University's Sleep Research Centre, said: "People have been puzzling for years about what the purpose of sleep is, as we know that only certain aspects of it have a recuperative value.

"It's only relatively recently that we've begun to understand the extent to which sleep helps you remember.

"What we know now is that sleep serves the function of a tidying process.

"The barin is highly organised when it comes to memories - it doesn't just chuck everything into the bottom drawer.

"Sleep is part of that proces of consolidation, perhaps because when we are asleep our brain is less distracted by all the noise and irrelevant information we come across when we are awake.

"It's as if the brain plays all the day's experiences in a type of weird fast-forward, and then sorts them out - saying that goes with this; I'm going to store that in this place.

"We form and store huge numbers of experiences in the head every day, and sleep seems to be the way the brain copes with it all."