The Hidden Door is an informative and interesting book about the variety and mysteries of dreams - but Elizabeth and Peter Fenwick have got the wrong mysteries. After more than a century of research, we still do not know why we dream, nor what it means to be conscious during sleep. These are the real mysteries, but the Fenwicks concentrate instead on the "mysteries" of dream telepathy, claims of the paranormal and excursions beyond the body.

Much of the book is highly informative, with many examples of dreams, visions and borderline states of consciousness. The history of early attempts to understand dreams includes not only the best-known stories of ancient Greek and biblical prophecies, but the birth of scepticism about the divine nature of dreams with Democritus and Aristotle. Psychoanalysis is covered simply and critically, and there are straightforward explanations of modern dream theories and the basic science of dreaming.

Many children wake in the night afraid and upset. We learn here the difference between nightmares, which are true dreams, and night terrors, which occur during non-dreaming sleep. Night terrors are a kind of confused arousal and, unlike nightmares, are rarely remembered by morning. Other phenomena of non-dreaming sleep include sleep walking and sleep talking. These cannot occur in true dreams because your muscles are paralysed to prevent you from wandering about. Lucid dreams, in which you know during the dream that you are dreaming, have been studied in the laboratory and shown to be true dreams. The pleasure of lucid dreams is compared with the horror of waking up unable to move, as in "sleep paralysis". This frightening experience may account for many scary visions and even for alien abduction experiences. Finally, some dreams are best described as mystical experiences, involving a world of light and peace, or experiences of oneness that can change a person's life.

There are fascinating examples of women's dreams during pregnancy and childbirth, but beware the sexist language throughout. There is surely no need to refer to humans as "man", and to make every therapist, patient, researcher and child with nightmares a male. Does it matter? Yes. Research shows that when we read we imagine concrete people - of one sex or the other - not abstractions.

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Creativity during dreams holds a special fascination and The Hidden Door describes many military decisions, scientific insights, and artistic creations that have been influenced by dreams. Here the Fenwicks are good at explaining the differences between true dreams, thinking during non-dreaming sleep and borderline states such as hypnagogia. So, for example, we learn that Salvador Dali trained himself to stay in a dozing state, using the hypnagogic imagery to inspire his painting, while Coleridge's poem "Kubla Khan" came to him in a true dream.

I particularly enjoyed the unmasking of some of these common myths. Coleridge's famous story of the poem's instant creation is not quite so convincing when you learn that in 1934 an autographed manuscript of the poem was found showing evidence of many earlier drafts. Then there is Kekule's discovery of the structure of benzene while watching atoms gambolling in a hypnagogic vision. Apparently he first mentioned the vision ten years after the event, possibly to avoid crediting other scientists with prior discovery. Nevertheless there is no doubt that dreams and hypnagogic images can be creative when hard work and intensive preparation have brought the mind to a state of readiness. But can the sleeping mind see at a distance, gain glimpses of the future or leave the sleeping body to travel in some other plane?

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Certainly many people believe so: 42 per cent claim to have had a dream that came true, and the Fenwicks describe many such dreams, from the most famous to the apparently trivial. To be fair, they do consider sceptical explanations, but without giving them much of a chance. For example, they suggest that some dreams "might simply be coincidence" but in at least one case conclude "it is easier to believe that the dream was precognitive." In fact, with over 60 million people in this country, you need only assume that each person has one dream of someone dying in their life to calculate that several times a year someone, somewhere in Britain will have a death dream that comes true the next day - just as someone has to win the lottery. If you are that person you will be convinced that your dream was psychic and understandably impressed - but the Fenwicks prefer to emphasise the mysteries that science cannot explain. They discuss laboratory work on extra-sensory perception in the Ganzfeld (a kind of sensory deprivation) without mentioning the recent discoveries of errors and possible fraud. They describe experiments in which chickens could apparently use the power of their minds to influence robots, without explaining their serious methodological flaws. And they describe the early dream telepathy experiments without making it clear that these experiments were criticised at the time and have not been successfully replicated. Their conclusion is that "even if the evidence that mind may directly affect mind and mind may directly affect matter is approaching a point where you would have to be the modern equivalent of a medieval flat-earther to reject it, science still has no theory to explain it". Poor science. Although Peter Fenwick is a scientist and author of some excellent research on sleep and meditation, the impression that comes from this book is that science is ignoring the big mysteries - that mind is something more than matter, and that dreams may hold the key to overthrowing modern "reductionist science".

One of the few scientific theories that tries to explain the mystery of dreams is Francis Crick's - that the brain's neural networks become overloaded with information during the day and have to be flooded with random activity during sleep to remove spurious associations, leaving the network ready to take on more the next day. The Fenwicks dismiss this with the comment that "if it had been put forward by anyone but a Nobel laureate the peer-reviewing process would have ensured that it remained just a dream". Yet what they replace it with does not provide any answers either. A lucid dream is like waking up inside a dream. The very existence of such awareness during sleep raises profound questions about the nature of self and consciousness, but the Fenwicks are content to explain it as the "ego-functions return into dream awareness". They talk of ideas "coming into consciousness", of "the pre-conscious mind" and "the dreaming mind" without once questioning what these terms might mean or what theory of consciousness underlies them. These are the real mysteries, but you will not find them explored behind this particular hidden door.

Susan Blackmore is senior lecturer in psychology, University of the West of England.