Continuing our series on the Big Science Questions, Stephen LaBerge looks at the role dreams play in our conscious and unconscious lives.

Every night we enter another world, the world of dreams. While we dream, we usually implicitly believe that we are awake. The mental worlds of dreams are so convincing that we mistake them for the "outside world". How can this be? Why does it happen? How do our day and night lives relate? And what is the origin and function of dreaming? Incredibly, there is little scientific consensus about answers to these questions.

The Oxford English Dictionary defines a dream as "a train of thoughts, images, or fancies passing through the mind during sleep". But that definition fails to capture the lived-in, experiential reality of dreams. Dreams are more accurately described as experiences - conscious events one has personally encountered. The essential criterion for consciousness is reportability: that we can sometimes remember our dreams shows them to be conscious mental processes. We live through our dreams as much as our waking lives. In these terms, dreaming is a particular organisation of consciousness.

Of course, that raises the question: what is consciousness? For me, it is the dream of what happens. Awake or asleep, your consciousness functions as a simplified model of yourself and your world, constructed by your brain from the best available information. During waking, the model is derived from external sensory input, which provides information about present circumstances, in combination with internal contextual, historical and motivational information. During sleep, little external input is available, and the model is constructed from internal biases. The resulting experiences are what we call dreams, the content of which is largely determined by what we fear, hope for and expect.

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There are two kinds of sleep: an energy-conserving state known as quiet sleep (QS), associated with growth, repair and restoration, and a very different state known as active sleep, REM or paradoxical sleep (PS). This is associated with rapid eye movements and muscular twitches, a paralysed body, a highly active brain and dreaming. Although PS is not the only sleep state in which people can dream, it provides the optimal conditions for vivid dreaming - a switched-on brain in a switched-off body.

It is frequently assumed that waking and dreaming experiences are distinct. Dreams are said to be characterised by lack of reflection and control and inability to act deliberately. But the evidence contradicts this. In recent studies comparing reports from waking and dreaming, my colleagues Tracy Kahan, Lynne Levitan, Phil Zimbardo and I found that, compared with waking experiences, dreaming contained public self-consciousness and emotion slightly more frequently, and deliberate choice slightly less frequently. But no significant differences between dreaming and waking were found for other cognitive activities, and none of the measured cognitive functions was typically absent or rare in dreams. In particular, nearly identical levels of reflection were reported in both states.

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I am not saying there are no differences between dreams and waking experiences. But they are more alike than different. For example, the dream world is much less stable than the waking world because dreams lack the stabilisation of an external structure - physical reality. One can violate the laws of physics and society in dreams without the usual consequences. But the absence of sensory constraint is the only essential difference. One might or might not know that one is dreaming and it would still be a dream.

It is likely, however, that PS evolved for purposes more basic than dreaming. Dreaming is perhaps something that humans do, and extract value from, but that did not evolve directly.

The distribution of PS across development and in the course of a night provides a clue to the most important functions of this sleeping state. PS is at its maximum level perinatally and in the last weeks of prenatal development when the brain is growing its vast networks of neural circuitry. The appealing idea that PS serves as an endogenous state for the unfolding of genetic programming has been proposed by several researchers. The percentage of PS gradually drops off throughout childhood, but does not completely disappear when brain growth stops in adulthood, implying that PS may serve another function. The fact that PS gradually increases across the night, reaching a maximum as the time of wakening approaches, suggests that it may prepare our brains for waking action; a sort of brain tune-up. These recurrent activations every 90 minutes or so throughout the night may also help consolidate new learning.

One of the most characteristic features of dreams is how difficult they are to remember. The average person dreams at least six times a night, but recalls them perhaps just once a week. The explanation of why dreams typically are forgotten rapidly again lies with evolution. Humans learn that dreams are distinct from other experiences by talking to other humans. Non-speaking animals have no way to tell each other how to distinguish dreams from reality. For them, explicit dream recall could cause potentially fatal confusion. The purpose of PS cannot therefore have anything to do with the explicit dream recall or interpretation. Nevertheless, since humans can tell the difference between dreams and waking reality, remembering dreams should do us no harm.

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Even if dreaming has no special biological function, dreams may play a specific role. They may increase variability in the nervous system. Dreaming may generate a range of behavioural schemas or scripts guiding perception and action from which to select adaptive fits to changing environments.

Our dreams seem so real that it is usually only when we wake that we recognise them as the mental experiences they are. But there is a significant exception: sometimes while dreaming we consciously notice that we are dreaming. This state of consciousness is referred to as lucid dreaming. During lucid dreams, one can reason clearly, remember the conditions of waking life and act voluntarily within the dream upon reflection or in accordance with plans decided before sleep - all while remaining soundly asleep.

Until recently, researchers doubted that the dreaming brain was capable of such a high degree of functioning and consciousness. In the late 1970s, our laboratory research at Stanford University proved that lucid dreams did occur during unambiguous sleep. Based on earlier studies showing that some of the eye movements of PS corresponded to the reported direction of the dreamer's gaze, we asked lucid dreamers to carry out distinctive patterns of voluntary eye movements when they realised they were dreaming. The prearranged eye-movement signals appeared on the polygraph records during uninterrupted PS, proving that the subjects had been lucid.

Subsequently, my colleagues and I began studies of the dreaming mind using lucid dreamers to carry out experiments in dreams. We learnt that the physiological effects on the brain and body of dream activities are nearly identical to the effects of experience in waking life. We found that time intervals estimated in lucid dreams closely match actual clock time, that dreamed movements result in corresponding patterns of muscle twitching, and that dream sex shows physiological responses similar to actual sexual activity.

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In addition to providing an effective way to carry out scientific explorations of dreaming consciousness and mind-body relationships, lucid dreaming offers considerable potential for other applications. These include aiding self-development, enhancing self-confidence, overcoming nightmares, improving mental health, facilitating creative problem-solving, and opening the mind to higher development. The broadest appeal of lucid dreaming is likely to be the fantasy-come-true of being able to have any imaginable experience.

Dreams have long been regarded as a wellspring of inspiration. August Kekule's dream of a snake biting its own tail inspired his discovery of the previously unsuspected ring structure of benzene. In the past, we have had little or no control over the occurrence of creative dreams. But it now seems possible that the fantastic and heretofore unruly creativity of the dream state might be brought within our conscious control by lucid dreaming. As Kekule urged his colleagues on the occasion of presenting his dream inspiration to a scientific convention in 1890: "Let us learn to dream."

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Stephen LaBerge is director of the Lucidity Institute, Palo Alto, California ( ). This essay is part of The THES Big Science Questions series, which will be published by Random House next year.

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