Most of us at some time have felt a sense of deja vu when we have seen or done something that we have dreamt about.

For some it has led to fortune, as in the case of Countdown contestant Ralph Lubkowski, who last week produced two perfect nine-letter words and later claimed that he saw the letters in a dream the previous night.

He is not the first person whose dream has quite literally come true.

Carol Stevens became a grandmother twice within hours when her two daughters gave birth on the same day - just as she had seen in a dream three years earlier.

At the time of the dream, one of her daughters didn't even have a boyfriend. ''I'm not psychic,'' she says. ''I'm just an ordinary mum who had a dream and then forgot about it.''

But can dreams actually predict anything - fortune or disaster?

According to the Association for the Study of Dreams, there are many examples of dreams that appeared to predict future events but were actually due to coincidence, faulty memory or an unconscious tying together of known information.

A few studies have been conducted on predictive dreams, as well as clairvoyant and telepathic dreams, but the results have been varied, as these kinds of dreams are difficult to study in a laboratory setting.

Every 90 minutes during sleep, we dream - for about 20 minutes or so at a time. Dreaming sleep is also known as rapid eye movement (REM) sleep, because the eyes move in bursts of activity similar to that in wakefulness when we scan our surroundings.

Over 100 years ago Sigmund Freud wrote The Interpretation Of Dreams, offering the theory that dreams are the disguised fulfilment of a repressed, infantile wish.

More recent research suggests that dreaming might be the brain's way of replaying events from the day before so that they are fixed in the memory for use later on.

European scientists established that the same regions of the brain which are active while we learn a new task are also active while we dream.

There have been many cases in which dreams have predicted life-changing fortune or disaster.

Antonia McMaster, from Manchester, believes she saw her husband Paul in a dream long before she met him. In her dream, though, he was dressed as a medieval knight in shining armour.

In another case, a plumber twice dreamed of digging up coins in a field, took his metal detector to the place he had dreamed about and found nearly 4,000 Roman coins dating back to 300AD.

And American grandmother Pearl Anderson dreamed she saw money gushing from a one-armed bandit, drove straight to the gambling city of Reno, Nevada and promptly won a million dollar slot machine jackpot.

Dreams have also been known to foresee impending doom.

For 14 years Caroline Harrison was tormented by a recurring dream of drowning at night. Then her nightmare almost came true when she was among a handful of survivors from a ferry disaster which killed up to 450 people off the island of Sumatra.

Research into predictive dreams suggests that premonitions do exist because some people have too many to be explained by consequence.

Some dreams cannot be explained, but there are some explanations for other unusual dreams.

For instance, if you dream of an accident involving a piece of equipment you use regularly then it might be worth having it checked, says Joan Hanger in The Little Book Of Dreams.

It may be that your unconscious has noticed some deterioration that could cause an accident.