by ROGER DOBSON, Daily Mail

The next time you're rudely awakened by restless children or deafening planes in a dawn chorus, remember this - for Penelope Cruz, nothing less than 15 hours will do.

The secret of her flawless skin and radiant complexion is, she says, a perfect night's - and morning's - sleep. It's the sort of slumber most of us can only dream about.

Cruz says: "Sleep is the best way to happiness and beauty. Once, my sister slept for three days straight. My record is 18 hours, though I don't usually sleep that long."

But can lying in bed for three-quarters of the day really help you to get the mind and body you crave, or should we do as Mrs Thatcher used to and survive on three or four hours a night - a technique, she claimed, that assured her strength of mind throughout the day?

"Fifteen hours of sleep is excessive by any standards. There is no evidence that taking an excessive amount of sleep like that is going to do you any good at all," says Professor Jim Horne, director of the sleep research centre at Loughborough University.

"In fact, the evidence suggests that taking excessive sleep is counter-productive because you are more likely to wake up depressed and miserable. Research also suggests that if you are already sad and miserable, actually reducing the time spent sleeping can be of benefit."

In fact, scientists still know very little about what constitutes the perfect night's sleep and why we seem to need so much of it.

Despite centuries of endeavour by some of the finest minds, the biological function of sleep remains a mystery, and precisely what goes on in the brain and body during it is also unknown.

As late as the middle of the last century, sleep was thought to be a time of inactivity when the brain switched off and took time out, but it's now known, thanks to modern scanning equipment, that the brain is highly industrious during sleep. The eyes may be closed, but behind the shutters there is a hiveof activity.

One of the most persistent theories down the centuries has been that sleep is needed to allow the body to rest. But research shows that sleep deprivation has little effect on the body.

"If you deprive people of sleep, nothing seems to go wrong physically. We can usually rest and relax quite adequately during wakefulness, and there is only a modest further energy saving to be gained by sleeping. When we go without sleep it is our brain and thinking and behaviour which is affected more than anything else,î says Professor Horne.

Sleep appears to be triggered by the discharge of particular nerve cells in the brain. These cells fire off messages telling the body to wind down and chemicals that cause drowsiness begin to circulate in the blood.

Once we are asleep, there are two different types, REM or rapid eye movement, which is the time for dreams, and deep sleep when there are no dreams.

The purpose of deep sleep,it's suggested, is to allow the brain to recover from the day's exertions,a time for it to regenerate and reorganise. It is thought thatfour hours is needed for this.

While the body and its vital organs are kept ticking over, the brain works off-line on a variety of housekeeping tasks, including remembering, prioritising and filing experiences of the previous waking period.

Hormones are also produced during sleep to carry out running repairs and to boost the immune system ready for the next day. The skin, too,gets a helping hand, and so do muscles.

Dr Neil Kavey, director of the sleep centre at Columbia Presbyterian Medical Center, New York, says that sleep improves muscle tone and skin appearance. "With adequate sleep,athletes run better, swim better and lift more weights. W e also see a difference in immune responses depending on how much someone sleeps.

Sleep is also the time when the body does most of its repair work, muscle tissue is rebuilt and restored. We know, too, for example, that growth hormone is secreted during sleep.î

Professor Horne says sleep must serve some vital function other than just energy conservation: "Whereas the body can relax and recover both when we are awake and asleep, the brain can't switch off during the waking time. The idea that you can rest your brain without going to sleep is nonsense.

"While you are awake, the brain is in a state of quiet readiness ready to respond. Even during sleep it only goes offline in the first four hours, when some kind of recovery seems to take place."

The importance of those first four hours may explain why some people can get by on a few hours' sleep. Research suggests the average adult needs seven to eight hours a night, though, according to the National Sleep Foundation, only a third of us manages that.

Disturbed or interrupted sleep is a common problem, although it seems to be normal for most people to wake once or twice a night. Sleep disturbance only becomes a problem if the sufferer feels tired during the day.

Insomnia is lack of quality sleep rather than lack of sleep, and sufferers - up to a third of the population at some time - may have unwanted awakenings,difficulty in falling asleep or daytime drowsiness.

T wo out of three people in Britain suffer from insomnia, mostly linked to stress and anxiety, according to one study. People heavily involved in their work and job are among those at higher risk, especially in the 25-35 age group.

And Sunday is the night of the week when we have the most sleep difficulties, possibly because the brain is already preparing for the week ahead.

So is the Cruz school of laziness any better for us? Professor Horne is not convinced: "It is all too easy to eat and drink well beyond our biological need - that second helping of enticing dessert, the delicious ice cream, another pint of beer ... the same applies to sleep," he says.

"Sleeping for nine or more hours a night is not necessarily beneficial to health. We have studied people who extended their sleep for m a ny nights,remaining in bed for 10 hours after lights-out and sleeping for as long as they wanted.

"They gained little and discovered distinct disadvantages - it took them longer to fall asleep, they woke up more during the night and found getting up in the morning to be no easier."