Science: What shall I dream tonight?

We can learn to control our own dreams - as well as deciding when to wake up.

Late for work? Forget that old excuse about the alarm clock not working - scientists have found that we can probably wake up at any time we like, simply by deciding to in advance.

Sleep, the embodiment of all that is not conscious, was assumed to be controlled by automatic hormonal mechanisms - known colloquially as "the body clock" - which are in turn dependent on genes, and on environmental influences such as the length of daylight hours. However, a study carried out at the University of Lubeck, Germany, shows that sleep can also be affected by the conscious brain. In effect this means that we can, if we try hard enough, decide when to wake up.

The researchers split a group of volunteers into two groups and, come nightfall, told one group that they would be allowed six hours' sleep, and the other that they would be allowed nine hours'. As the volunteers slept, the researchers measured their levels of adrenocorticotropin, a stimulating hormone that is released shortly before spontaneous awakening. In both groups the levels of adrenocorticotropin started to rise about an hour before the time they had been told they would have to get up.

The three-hour difference could be explained only by what they had been told when they were awake. This suggests that anticipation - a conscious activity - can pervade sleep and influence bodily mechanisms that were previously thought to be entirely unconscious.

This intertwining of the conscious and unconscious mind has been revealed by many studies, but most of them demonstrate how the unconscious mind affects consciousness, rather than vice versa. French scientists, for example, recently found that people can identify a written word as familiar even though they have previously seen it for too short a time to register it consciously. Faces can similarly be recognised subliminally, and unconscious recognition has been shown to influence whether or not one person finds another appealing.

The influence of the conscious mind on the unconscious is more difficult to investigate because an unconscious person is out of contact and unable to indicate what - if anything - is going on in their head. There is, however, a curious condition in which a person is partly conscious and partly asleep, and in which the conscious part of the mind is able to observe some of its unconscious mechanisms. This state is known as lucid dreaming.

Lucid dreaming is generally confined to the "weird and wacky" school of science reporting, and tends to be classified with parapsychology or even psychic phenomena, so few serious researchers have investigated it, and those who have have received scant support. This is unfortunate, because lucid dreaming is a unique tool for exploring consciousness.

It occurs when a person "wakes up" while dreaming - but, instead of clicking back into the real world, continues to dream. The fantasies concocted by the dreaming brain appear to be as solid as the real world, but, as in all dreams, the content is bizarre. Because the dreamer's brain is in full waking mode, he or she realises that such effects must be hallucinations.

This odd state comes about when parts of the brain wake up and other parts stay asleep. During normal dreaming, most sensory input from the outside world is blocked off but the brain continues to generate sights and sounds by drawing on old sensual memories and binding them into complex, often crazy narratives - dreams. If dreams happened while we were awake we would instantly recognise them as unreal because the frontal lobes of the brain, which are active during normal waking, constantly scrutinise the world in a critical way, leaping on anything odd and subjecting it to rational analysis. While we sleep, however, the frontal lobes power down, so when a dream appears we fail to recognise that the things happening in it are daft or impossible. The frontal lobes are also responsible for producing a stable sense of identity. This is why, when they are switched off in dreams, we may turn into other people, or even be two people at once.

In lucid dreaming, what seems to happen is that the frontal lobes come back "online", clicking us back into a normal waking frame of mind. But the rest of the brain continues to dream, throwing up typically barmy scenarios. Another aspect of sleep - paralysis - also continues, so the dreamer is unable to move, even though the "dream body" continues to move and be felt.

A first lucid dream can be frightening, but once people are used to them they usually enjoy them. Unlike ordinary dreams, lucid ones can be controlled consciously, and experienced lucid dreamers learn to create marvellous fantasies and physical sensations, such as leaving the body and flying. It is possible to conjure up a true-as-life image of anything - a paradise island, the view of Earth from outer space, a dead friend; the content is constrained only by what the brain has to draw on in the way of memories and ideas. If you have never seen the view of the Earth from outer space, the sight you will get will be at best a guess - but it will seem absolutely convincing. The only glitches tend to involve machines, which never work properly in lucid dreams, and light levels, which fluctuate in a totally unearthly way.

Lucid dreams are often imbued with an other-worldly atmosphere. Sometimes they seem intensely sinister, at other times they are accompanied by a feeling of ecstasy. The uncanny effects - especially "out-of-body experiences" and ghostly visitations - have led many to misinterpret the condition as supernatural. Until recently, many psychologists and sleep researchers denied that it even existed.

Although lucid dreaming gives us an insight into the back rooms of our brain, it does not offer much scope for conventional research because the dreamers cannot take an active part in experiments. They can't react to events in the waking world because sensory input from outside is blocked, as in normal sleep. And although they know that they are in a lab being watched by people keen to know what is going on, sleep paralysis prevents them from talking or signalling. If they try to raise a hand, all that happens is that their dream hand is raised.

There is, though, one small window through which the dreamer can communicate with those in the outside world. Sleep paralysis does not affect the eye muscles, so when dreamers go lucid they can move their eyes at will. By sweeping their eyes back and forth a prearranged number of times, lucid dreamers have signalled to researchers the start of their dreams, and answered simple yes/no questions about what they are experiencing. By this means, a few basic things have been established about lucidity: for example, it nearly always arises out of REM (rapid eye movement) sleep, and usually starts quite late in the sleep cycle.

Most people are likely to experience a fleeting lucid dream at least once, and two in ten say they "go lucid" at least once a month. Professor Stephen LaBerge, of Stanford University, who runs the "Lucidity Institute" in California, says that anyone can learn to do it. The trick is to tell yourself repeatedly while awake that when you next go to sleep you will watch out for oddities in your dream, and "wake up" when you notice them. Training yourself to recall your dreams each morning also helps.

Until it happens, it is difficult to imagine how these simple conscious exercises can alter what happens while you are unconscious. But, as with the "alarm clock" exercise, it seems that the machinations of your conscious mind can pervade and influence your sleeping brain.

The boundaries between consciousness and oblivion are more blurred than we may think.

Rita Carter's latest book, `Mapping the Mind', is published by Weidenfeld and Nicolson, price pounds 25