To sleep, perchance to think

Man sleeping
Deep in thought: there is much more to REM than meets the eye

The meaning and cause of dreams has fascinated scientists for centuries. The brain is never idle, says Jim Horne

Dreams can be delightful and inspirational. Leonardo da Vinci attributed many of his inventions to them and wrote: "why does the eye see a thing more clearly in dreams than the imagination when awake?"

Friedrich Kekule, pioneer of organic chemistry, attributed his inspiration for the benzene ring to a dream in which a snake swallowed its tail. Robert Louis Stevenson frequently had vivid dreams, and his The Strange Case of Dr Jekyll and Mr Hyde came from one.

The link between eye movements and dreaming was first suggested a long time ago, with one of the earliest accounts appearing in 1892. We refer to this dream state as Rapid Eye Movement (REM) sleep.

But although dreaming is most vivid and intense during REM sleep, milder, more reflective and less fantastic forms of dreaming occur throughout much of the rest of sleep - and we don't always dream during REM sleep.

In fact, there is much more to REM sleep than meets the eye. It's more like wakefulness and maybe we should view REM sleep not as true sleep but as a type of wakefulness, or even non-wakefulness, rather like screen-saver mode on a computer. The brain is not really unconscious during REM sleep.

Whispering a familiar name or some other meaningful sound will usually produce an instant awakening. Whereas a fairly loud and meaningless noise, even a train thundering past (if you live beside a railway) will be ignored, providing that it's not annoying.

Annoyance or the emotional significance of the sound is particularly important in determining whether we wake up from REM sleep. Even in our enlightened age of the equality of the sexes, the crying baby is more likely to wake up mum rather than dad, and from REM sleep. Low-flying aircraft may well only wake up dad, who gets particularly annoyed by these flights, not mum, who may have other matters to worry about.

Emotive external sounds will quickly awaken us from REM sleep, so how is it that dreams, with all their emotional content, do not normally wake us up, except from the more frightening nightmares?

The dreamer seems to be emotionally numbed and protected from the emotional content of the dream. There are little or no increases in heart and breathing rates, or in blood pressure (although these do become more irregular) during REM sleep. Moreover, blood levels of the fight-or-flight and stress hormones, adrenaline and cortisol remain unchanged.

How all this is achieved is not clear, although important regions of the brain controlling emotional behaviour, such as the amygdala, almond-shaped structures, are very active in REM sleep, which may, in part, be due to their blocking of these emotions. The amygdala is probably the guardian of REM sleep as here is where the sleeping brain's main watchkeeper resides, keeping us vigilant to external threats.

Other evidence that suggests vigilance during dream sleep comes from electrical activity accompanying each burst of rapid eye movements. Initiated from deep in the brain, PGO waves (so named because they are from three brain areas: the Pons, Geniculate thalamus and Occiput areas) vary in intensity just as do the eye-movement bursts. Both are accompanied by small, almost unnoticeable twitches of the fingers and toes.

These waves are largely absent from the rest of (non-REM) sleep, but something very similar occurs in wakefulness whenever an animal is alerted to a new stimulus, when it will prick up it ears and look towards whatever is causing the interest (in humans, though, the waves are difficult to detect without the aid of electrodes implanted in the brain).

In REM sleep, PGO waves are not usually in response to some external stimulation, such as noise, but are spontaneous, unlike those in wakefulness. Nevertheless, they bombard the cortex as if it was being stimulated externally, and as REM sleep is so prolific in the baby before birth, these waves probably help mature its developing cortex, to make up for the dull confines of the mother's uterus. Something similar continues to happen during adult REM sleep.

Our cortex, the rind of the brain where consciousness resides, probably tries to make some sense of all these weird pseudo-sensations, by creating imagined events around them, through dreaming. The disjointed nature of dreams, leaping from one unexpected theme to another, reflects this.

Not all of the cortex is involved in dreaming, though, as the frontal cortex, which in wakefulness helps us sustain attention to a particular event and ignore distractions, either can't cope with all this distraction during REM sleep and shuts down, or purposely switches off so that it can continue to rest and recover.

For whatever reason, it means that the rest of the cortex is freed from the frontal region's usual constraints, to go off and play, so to speak. Hence the unfettered distractions and leaps of imagination in dreams, which makes them so unpredictable and fascinating.

What is fairly clear is that we dream as we think - dreams are a surreal pastiche of what we have recently encountered and thought about while awake. Why people want to place some meaning into dreams or interpret them is beyond the comprehension of many scientists, and probably those who write about the meaning of dreams. There is usually greater fantasy in the mind of the dream interpreter than the dream to be interpreted.

Viewing dreams as portents of the future has been a popular activity. In ancient Egypt, priests would conduct their interpretations as part of sleep therapy conducted in sleep temples. An ancient papyrus, the Dream Book, from the era of Ramses the Second (1275 BP), exists in the British Museum, providing numerous dream examples and interpretations. People with troubled minds would spend the night in these special temples, where priests would try to influence sleep with suggestions, in the hope of provoking special dreams sent by the gods. In fact, Imhotep, the architect of the first step pyramid and a famous physician, advocated this practice. After his death, he was worshipped as a god, in sleep temples built in his honour.

Maybe, in certain situations, dreams can be of help in understanding the pressures confronting patients with severe mental problems, and, to some extent, Freud may have been correct in saying that dreams are the "royal road to the unconscious". But it is crucial to know the patient before any interpretation can begin, and there is no evidence to support the notion of universal symbols in dream imagery, common to everyone. Dream dictionaries and schemes for dream analyses by mail, where there is no rapport between the dreamer and interpreter, must be treated with the greatest of suspicion.

  • Prof Jim Horne is director of the Sleep Research Centre, Loughborough University. This is an edited extract from his new book, 'Sleepfaring: A Journey through the Science of Sleep' (Oxford University Press), which is available for £12.99 + £1.25 p&p. To order, please call Telegraph Books on 0870 428 4112
  • He will discuss his book at the Cheltenham Science Festival, which runs from June 7 to 11. For details of his lecture, and those by other well-known popularisers, such as Kathy Sykes, Sir Christopher Frayling, Robert Winston, Susan Greenfield, Nick Ross, Terry Waite and Quentin Cooper, see or ring the bookings line, 01242 227979