Why do we have dreams and what do they mean? These questions have for centuries been the subject of a debate that has recently become the center of a heated controversy. In one camp we have a number of prominent scientists who argue that we dream for physiological reasons alone and that dreams are essentially mental nonsense devoid of psychological meaning: "A tale told by an idiot, full of sound and fury, signifying nothing." The idea that dreams are nothing more than "meaningless biology" sounds absurd and rather blasphemous to the opposing camp, a coalition of Freudians and other dream workers committed to the view that we dream for psychological reasons and that dreams always contain important information about the self or some aspects of one's life which can be extracted by various methods of interpretation. This camp takes its credo from the Talmudic aphorism that "an uninterpreted dream is like an unopened letter." There is also a third camp occupying the middle ground, that believes both of the extreme positions on the function and meaning of dreams to be partly right and partly wrong. Its proponents argue that dreams may have both physiological and psychological determinants, and therefore can be either meaningful or meaningless, varying greatly in terms of psychological significance.
This middle position is where I find myself most comfortable. I agree with Sir Richard Burton that
"Truth is the shattered mirror strown in myriad bits; while each believes his little bit the whole to own."
Perhaps, however, we may be able to put together enough of the pieces to reflect the reality of the dream reasonably well. Although people have argued for many centuries over whether dreams represent the addle-minded children of an idle brain, the heaven-sent embodiment of wisdom, or something in between, we will confine our discussion to "scientific" theories of dreaming at least as modern as the 20th century. So then, let us start with Dr. Sigmund Freud.
If we are to understand Freud's view of the dream, we need to consider his concept of the dreamer's brain. We know today that the nervous system contains two types of nerve cells (excitatory and inhibitory). Both types discharge and transmit electrochemical impulses to other neurons. Both do this spontaneously, without any kind of outside stimuli, as well as when they themselves receive excitatory impulses from other cells. However, one critical difference between these two types of neurons is that one type, called "excitatory" transmits impulses to other neurons which causes increased nervous activity or "excitation" in them. The other type of neurons is called "inhibitory," because they send messages to other neurons that cause decreased activity or "inhibition." The human brain is constructed of an unimaginably complex network of intricate interconnections between billions of each type of neuron. Generally, the inhibitory neurons play a more important role in the higher functions of the brain.
Before developing his theory of dreams, Dr. Freud had intensively studied neurobiology. But, in his time, only the process of excitation had been discovered; the process of inhibition was not yet known. Based on the assumption of a completely excitatory nervous system, Freud reasoned that nervous, or in his terms, "psychic," energy could therefore only be discharged by means of motor action. This meant that once you got a notion in your head, it was doomed to run around in there forever until you finally decided to do something about it. Or, alternatively, until it found a way to trick you into unconsciously expressing it in some unintended action like the famous "Freudian slip."
This older view of the nervous system has been caricatured as a "cat on a hot tin roof" model, "with the persistent internal drives generating blasts of energy that keep the ego and conscious system in frenzied movement."  We know today that a nervous system of this sort, if it could exist at all, would erupt into uncontrolled seizure activity. However, given the state of knowledge of his time, Freud's view of the unconscious mind as a cauldron seething with socially unacceptable impulses and desires appears perfectly reasonable; and likewise, from it his theory of dreaming can readily be seen to follow.
Let us imagine what might have happened if you were somehow able to ask the master himself why you had a particular dream. Dr. Freud, we may speculate, might have answered something like this:
In the first place, we may be sure that something happened to you a day or two before the dream and that this "day residue"--as we call it--stirred up one of the many repressed wishes that you try to keep closeted away in your unconscious. But, when you drifted off to sleep with no other wish in your conscious mind than to sleep, you withdrew your attention from the external world, setting the stage for your day residue and associated unconscious wish to step forward, demanding satisfaction. All this requires the cooperation of the chief executive of your conscious mind, the ego. But because your pair of suppliants were not, let us say, 'dressed in a socially acceptable manner,' they were at first denied admission to your conscious mind. And that was as it should be! It is the special function of the gate-keeper to prevent unruly and unacceptable impulses, memories, or thoughts from disturbing your ego's conscious mind. The gate-keeper, which we psychoanalysts call the 'censor,' is able to do his job with the help of a big stick we call 'repression,' by means of which these impulses, memories, and thoughts which conflict with personal and social standards of behavior are banished from the conscious mind, along with the painful emotions and memories associated with the conflict. Since the repressed contents cannot be banished entirely, they settle to the bottom of your unconscious mind, where they simmer and seethe like a witches' cauldron.
But now and then, by the power of association, the events of the preceding day, in the form of day residues, dredge up these repressed wishes. Naturally, they seek a way to even partial fulfillment. That is what your day residue and repressed wish were doing, knocking on the door of the ego. However, after the censor threw them out, the vulgar pair, knowing nothing of manners, continued to clamor for admission, raising such a rumpus as to threaten your precious sleep and thereby frustrate your ego's only conscious wish. Fortunately, you were able to continue to sleep, thanks to your dream doing its job. As we say, 'dreams are the guardians of sleep.' Across the border, in your unconscious mind, a special process that we call 'dream-work' constructed a disguise for your repressed wish, out of 'acceptable' imagery linked to it by association. Thus transformed into a superficially presentable image, your wish was able to get by the censor and find expression in the your dreams.
"And that is why," Dr. Freud might well have said, "you had that dream; and please note," he might not have been able to resist continuing, "that your dream killed two birds with one stone: while preserving your sleep, it also allowed the discharge of one of your repressed instinctual impulses. That all this was a good thing seems undeniable;" Sigmund Freud might have concluded, "I need hardly add we regard it as axiomatic that the nervous system obeys the 'nirvana' principle, forever seeking the reduction of tension and the ultimate cessation of action."
In some ways, of course, this aspect of psychoanalysis has strong parallels with Buddhism and other Eastern doctrines. But that brings us no closer to answering your original question, and you might well ask: "But what did my dream mean? Or was it just nonsense?" In that case, Freud would have probably explained that "Every dream has some hidden meaning; since the manifest content of the disguised dream (the dream itself) was the result of the dream-work's transformations of the undisguised wish (the latent content of the dream). Therefore, in order to interpret your dream, it should simply be necessary to reverse the process. Since the dream disguised the latent content with images closely associated with the original wish, we can uncover the hidden message by reasoning backward from the image through a process of interpretation known as "free association." If you had dreamed, let us say, that you were locking a door, Dr. Freud would ask you what was the first thing that comes into your mind in connection with the work "lock?" If you said, "key," Freud would continue, "key?" And perhaps you would reply, "tree." This, as you can see, might go on forever, except that Freud would probably have interrupted the process at this point and drawn on his knowledge of dream symbolism (key in lock...!) explaining that your dream expressed a wish to engage in sex!
In other words, Freud believed that the function of dreaming was to allow the discharge of repressed instinctual impulses in such a way as to preserve sleep, and that the instigating force causing dreams to occur was always an instinctual, unconscious wish. Dr. Freud considered these unconscious wishes to be predominantly sexual in nature. In "Introductory Lectures on Psycho-Analysis," he wrote: "Though the number of symbols is large, the number of subjects symbolized is not large. In dreams those pertaining to sexual life are the overwhelming majority...They represent the most primitive ideas and interests imaginable."  In any case, in so far as the instigating force behind every dream was an unconscious wish--whether sexual or otherwise--it follows from Freudian theory that every dream contains meaningful messages in disguised form: the original wish or "dream thought." The fact that all dreams contained unacceptable and unpleasant wishes explained why dreams are so regularly and so easily forgotten. This was because, reasoned Freud, they were (deliberately) repressed: blacklisted by the ego and sent by the censor to the bottom of the swamp of the unconscious.
We know today, thanks to the last 30 years of dream research, that dreams are not instigated by wishes or other psychological forces, but by a periodic or automatic biological process: REM sleep. If dreams are not triggered by unconscious wishes, we can no longer assume that these wishes play any role in dreams at all; and even worse for the Freudian concept of meaning, we can no longer automatically assume that every--or even any--dream has meaning! This is not all of the news recent neuroscience has for Freud; but let us save the bad news for the next section. The good news for Freud is this: every period of dreaming sleep is accompanied by sexual arousal, as indicated in males by erections, and in females by increased vaginal blood flow. Had Freud lived to hear of this phenomenon, he would have almost certainly regarded it as a complete vindication of his belief that at the bottom of every or almost every dream was-- sex!
In 1977, Drs. Allan Hobson and Robert McCarley of Harvard University presented a neurophysiological model of the dream process that seriously challenged Freud's theory on virtually every point. In a paper they published in the American Journal of Psychiatry entitled "The Brain as a Dream State Generator: An Activation-Synthesis Hypothesis of the Dream Process," they suggested that the occurrence of dreaming sleep is physiologically determined by a "dream state generator" located in the brain stem. This brain stem system periodically triggers the dream-state with such predictable regularity that Hobson and McCarley were able to mathematically model the process to a high degree of accuracy. During the REM periods produced when the dream- state generator is switched "on," sensory input and motor out-put are blocked, and the forebrain (i.e., the cerebral cortex, the most advanced structure in the human brain) is activated and bombarded with partially random impulses generating sensory information within the system. The activated forebrain then synthesizes the dream out of the internally generated information, trying its best to make sense out of the nonsense it is being presented with."
"The primary motivating force for dreaming," emphasize Hobson and McCarley, "is not psychological but physiological since the time of occurrence and duration of dreaming sleep are quite constant, suggesting a pre-programmed, neurally determined genesis." They see the major drive for the dreaming process as not only automatic and periodic but apparently metabolically determined; of course, this conception of the energetics of dreaming flatly contradicts the classical Freudian notion of conflict as the driving force for dreams.
As for the "specific stimuli for the dream imagery," they continue, these appear to arise from the brain stem and not from cognitive areas of the cerebral cortex. "These stimuli, whose generation appears to depend upon a largely random or reflex process, may provide spatially specific information which can be used in constructing dream imagery." Hobson and McCarley argue that the bizarre distortions in dream content attributed by Freudians to the disguising of unacceptable content by the "dreamwork" probably have a simpler neurophysiological explanation instead: such bizarre features of dreams as the condensation of two or more characters into one, discontinuous scene shifts, and symbol formation may merely directly reflect the state of the dreaming brain.
"In other words," the Harvard neurophysiologists argue, "the forebrain may be making the best of a bad job in producing even partially coherent dream imagery from the relatively noisy signals sent up to it from the brain stem. The dream process is thus seen as having its origin in sensorimotor systems, with little or no primary ideational, volitional, or emotional content. This concept is markedly different from that of the 'dream thoughts' or wishes seen by Freud as the primary stimulus for the dream."
Hobson and McCarley view "the elaboration of the brain stem stimulus by the perceptual, conceptual, and emotional structure of the forebrain," as primarily synthetic and constructive, "rather than a distorting one as Freud presumed." According to the Activation-Synthesis model, "best fits to the relatively inchoate and incomplete data provided by the primary stimuli are called up from memory...The brain, in the dreaming sleep state, is thus likened to a computer searching its addresses for key words. Rather than indicating the need for disguise, this fitting of...experiential data to [genetically programmed] stimuli is seen as the major basis of the 'bizarre' formal qualities of dream mentation." Scoring one more point against Sigmund Freud, they add that "there is, therefore, no need to postulate either a censor or an information degrading process working at the censor's behest."
Hobson and McCarley see our usual poor ability to recall our dreams as reflecting "a state-dependent amnesia, since a carefully effected state change, to waking, may produce abundant recall even of highly charged dream material." So that if you are rapidly awakened out of REM sleep, you are likely to remember dreams that you would otherwise be just as likely to forget. Hammering a final nail into the coffin containing Freud's theory of dreams, they write: "There is no need to invoke repression to account for the forgetting of dreams."
As was only to be expected, Hobson and McCarley's paper stimulated counter attacks from the psycho-analytic establishment, which responded that Freud's neurological models were in no way crucial to his psychological theories. In the view of Dr. Morton Reiser, Chairman of the Department of Psychiatry at Yale University, and a past president of the American Psychoanalytic Association,
McCarley and Hobson overextend the implications of their work when they say it shows that dreams have no meaning. I agree with them that their work refutes Freud's idea that a dream is instigated by a disguised wish. Knowing what we do now of brain physiology, we can no longer say that. The wish may not cause the dream, but that does not mean that dreams do not disguise wishes. The brain activity that causes dreams offers a means whereby a conflicted wish can give rise to a particular dream. In other words, wishes exploit--but do not cause--dreams. 
The degree of controversy stimulated by the Hobson and McCarley paper was truly remarkable. An Editorial in the American Journal of Psychiatry a year later stated that the Harvard paper "provoked more letters to the Editor than the Journal had ever received before." Unexpectedly, what seemed to be stirring so many people up was not Hobson and McCarley's treatment of Freud, but their treatment of the dream. The view that "dreams were after all merely the senseless, random accompaniment of the autonomous electrical activity of the sleeping Central Nervous System" did not sit well with many dream researchers, to say nothing of therapists and other dream workers accustomed to putting dreams to a variety of practical uses.
Anybody who has ever awakened from a dream, exclaiming with delight, "what a wonderful plot that was!" will know from their own experience that at least sometimes, dreams are much more coherent than would seem to be suggested by Hobson- McCarley's model of "the forebrain making the best of a bad job in producing even partially coherent dream imagery from the...noisy signals sent up to it from the brain stem." In the view of many scientists and dream researchers including myself, the fact that dreams can be such superbly coherent and entertaining stories is an indication of the need to concede to the forebrain at least an occasional or partial degree of control during dreaming. How could we construct such extended and coherent dream plots if the higher brain centers are limited to mere improvisation with whatever unrelated props, people, and scenes that the "noisy signals" from the brain stem happen to kick upstairs? The dream Hobson and McCarley seem to envision would be like "And now for something completely different!" every minute or two. The fact that we are able at times to produce dreams that are so wisely and elegantly constructed that they can and do serve us as teaching stories suggests that higher order mental functioning must in some way be able to influence the lower order functioning of the dream state generator.
The phenomenon of lucid dreaming suggests even more strongly the influence of the cerebral cortex on the construction of dreams. For if your dreams were nothing more than the results of your forebrain "making the best of a bad job in producing even partially coherent dream imagery from the relatively noisy signals sent up to it" from your brain stem, how would you be able to exercise volitional choice in a lucid dream? How would you be able to carry out a previously planned dream action? How would you be able to deliberately decide to, let us say, open a door to see what you might find there?
Lucid dream reports abound with counter examples, showing that dreamers can at times have their own feelings, intentions, and ideas. When dreamers realize that they are dreaming, they often experience a feeling of exhilaration. This feeling is more like a response to a higher order perception, rather than to a random brain-stem stimulus. As for intentions and ideas, when dreamers attain lucidity, they typically remember intentions about what they wanted to do in their next lucid dream, and can remember ideas in the form of ideals and principles of behavior such as "face your fears," "seek positive outcomes," or "remember your mission." Our oneironauts routinely make use of this last principle when sleeping in the laboratory.
Finally, if all of the eye movements of REM sleep are randomly generated by a madman in the brain stem, how are lucid dreamers able to voluntarily execute eye movement signals in accordance with pre-sleep agreement? Of course, the answer to all of these rhetorical questions is that the Hobson-McCarley hypothesis cannot be the whole story. On the contrary, I believe Hobson and McCarley are right about much of what they say about physiological determinants of the form of dreams; it is evident that dreams also have psychological determinants, and therefore any satisfactory theory of dream content ought to include both. It also ought to explain why and under what conditions dreams are sometimes coherent, brilliantly witty narratives, and in other conditions, incoherent ravings. And why in some dreams are we deluded and in others lucid? And why are some dreams profoundly meaningful and others pointless nonsense?
As for meaning and nonsense, the Activation-Synthesis model of dreaming seems willing to completely disregard the possibility that dreams could have any intrinsic or interesting meaning whatever. Given the "forebrain making the best of a bad job..." from the random signals sent up from the brain stem, the most we could reasonably expect if this were the case is what is called in computer terms "GIGO," an acronym for "Garbage In, Garbage Out." Hobson, at least, seems to say as much in a recent interview: "Dreams are like a Rorschach inkblot. They are ambiguous stimuli which can be interpreted any way a therapist is predisposed to. But their meaning is in the eye of the beholder--not in the dream itself."  I can hear it now: a psychiatrist asks a patient, "What does this dream make you think of?" And receives the reply: "An inkblot!"
Among the psychophysiologically minded dream researchers a major criticism of the Activation-Synthesis model was that it was essentially a one-way street, allowing traffic only to proceed upward from brainstem to forebrain (from lower mental function to higher mental function). But the way the brain is actually put together would require a two-way street, allowing forebrain control of brain stem activation, and therefore allowing higher cortical functions such as thinking and deliberate action to influence the dream. This is the same criticism I just made regarding the inability of the Hobson and McCarley model to deal with lucid dreaming.
Some sleep and dream researchers argued that the Activation- Synthesis model missed the central question about dreaming altogether. According to Dr. Milton Kramer of the University of Cincinnati, Hobson and McCarley's approach is "not central to the functional problems of dreaming. When it comes to dreams, two things are important--meaning and function. Do dreams enlighten us about ourselves? Will they make us smarter, change our personality, change our mood, solve our problems, have an application to our daily lives?" Kramer concluded that "I think the essence of dreams is psychological. It's all very well to find in dreams that a person is walking. The important questions are, 'Where is he walking? Why is he walking there?' Those are the continuing mysteries of dreams and that is what we want to know." 
So how does the Activation-Synthesis model measure up if we use Kramer's two criteria: meaning and function? As to the meaning of dreams, in Hobson and McCarley's model there is none. Regarding function, Hobson has offered a possible function of the dream state:
A crude analogy to computers helps to make a point even if it may violate the reality of brain function: Every information processing machine has both hardware and software components. To create a nervous system, the genetic code must program both a structural blueprint and a set of operating instructions. To maintain the neurons it would make sense to utilize a standard set of operating instructions to activate and test the system at regular intervals. From an intuitive point of view, it is appealing to consider REM sleep as the expression of a basic activity program for the developing CNS that would ensure the functional competence of neurons, circuits, and complex activity patterns before the organism was called upon to use them. It would be particularly important for such a system to have a high degree of reliability in both time and in space. These features are to be found in the periodicity and duration constancy of REM and in the stereotyped nature of the activity. 
Elsewhere, Hobson elaborates:
I believe that dreaming is the (sometimes outward) sign of a genetically determined, functionally dynamic blueprint of the brain designed to construct and to test the brain circuits that underlie our behavior-including cognition and meaning attribution. I also believe that this test program is essential to normal brain-mind functioning but that you don't have to remember its products to reap its benefits. 
In a paper published in 1983 in the British journal Nature, Nobel Laureate Francis Crick (one of the team that cracked the genetic code and unraveled the mystery of DNA) and co-author Graeme Mitchison proposed that the function of dream sleep
is to remove certain undesirable modes of interaction in networks of cells in the cerebral cortex. We postulate that this is done in REM sleep by a reverse learning mechanism, so that the trace in the brain of the unconscious dream is weakened, rather than strengthened by the dream. 
That, in a nut-shell, is their "reverse-learning" theory of dreaming.
Crick and Mitchison's theory is derived from two basic hypotheses: the first is that the cerebral cortex, as a completely interconnected network of neurons, "is likely to be subject to unwanted or 'parasitic' modes of behavior, which arise as it is disturbed either by the growth of the brain or by the modifications produced by experience."
Their second hypothesis is even more tenuous than the first: it proposes that if these hypothetical 'parasitic' modes of neuronal activity do in fact exist, then it might be that they "are detected and suppressed by a special mechanism" hypothetically operating during REM sleep. This mechanism is described as having "the character of an active process which is, loosely speaking, the opposite of learning." Crick and Mitchison call this hypothetical process "reverse learning" or "unlearning," and explain that it "is not the same as normal forgetting" and that "Without it we believe that the mammalian cortex could not perform so well."
"The mechanism we propose," write Crick and Mitchison, drawing on the Hobson-McCarley conception of the neurophysiology of dreaming,
is based on the more or less random stimulation of the forebrain by the brain stem that will tend to excite the inappropriate modes of brain activity...especially those which are too prone to be set off by random noise rather than by highly structured specific signals. We further postulate a reverse learning mechanism which will modify the cortex...in such a way that this particular activity is less likely in the future...Put more loosely, we suggest that in REM sleep we unlearn our unconscious dreams. "We dream in order to forget."
To reiterate: what they are suggesting is that everything that happens in any of your dreams is being actively unlearned by your brain--that is why you are dreaming about it: merely "in order to forget it."
What exactly does this mean? According to the reverse- learning theory, when we remember our dreams we are re- learning exactly what we were trying to unlearn. This would seem to represent at least a partial failure of the reverse- learning mechanism, and "one might wonder what effects its failure might have." Crick and Mitchison suggest that complete failure (remembering all of one's dreams) might lead to "grave disturbances-a state of almost perpetual obsession or spurious, hallucinatory associations..." A partial failure (remembering several dreams a night) "should produce unwanted responses to random noise, perhaps as hallucinations, delusions, and obsessions, and produce a state not unlike some schizophrenias."
Crick and Mitchison's motto is, "we dream in order to forget." Well, maybe they do for all I know. Unfortunately, they go further than that, seeming to feel that it would be better for all of us to learn to forget our dreams: "In this model," they write, "attempting to remember one's dreams should perhaps not be encouraged, because such remembering may help to retain patterns of thought which are better forgotten. These are the very patterns the organism was attempting to damp down."
Certainly, if the reverse learning model were followed to its logical conclusion, it would seem to call for the shut-down of all psychological analysis of dreams, all attempts at remembering and interpretation of dreams, in fact the complete shutdown of the dream work industry. Fortunately, it appears that there is absolutely no direct evidence for "unlearning" during REM. In fact, there doesn't appear to be even any evidence for "unlearning" of any kind in any state, in any living organism, anywhere. "Unlearning" as it now exists is only a hypothetical concept, perhaps of some relevance to computers, but there is no proof that it has any application to human beings. In fact, Crick and Mitchison admit "A direct test of our postulated reverse learning mechanism seems extremely difficult." 
There is, in short, no convincing argument for this theory. It just might be true or partially true, but until direct evidence supporting it is brought forward, it must be viewed as an unlikely possibility. Even if there were some substance to reverse-learning theory, Crick and Mitchison's conclusions about the desirability of dream recall are not necessarily correct. On the contrary, the strongest argument against the theory may be the catastrophic effects they predict to result from even partial failure of the reverse-learning mechanism. Certainly, people who habitually remember their dreams do not seem any more prone to "hallucinations, delusions, and obsessions" than are people who habitually forget their dreams. Similarly, if the unlearning theory were true, dream deprivation would interfere with the "reverse-learning" process, producing disastrous effects. However, people have been deprived of REM sleep for many nights and in some cases years without showing any signs of mental breakdown. So for any of you dreamers concerned about whether you may be messing up your mind by remembering your dreams, I would suggest that you need not worry!
Let us return to the question with which we began this chapter: "Why do we dream?" Though we have considered only a few here, there are many answers that could be and have been proposed to this question. But we can justly rule out in advance any theory of the meaning and function of dreams that does not make as much sense when applied to the dreams of a tree shrew or a whale as to the dreams of a hairless speaking primate--meaning us! Whatever the explanation for dreaming must be, we must dream for the same reason that all mammals have dreamed for more than a hundred million years. Then, why do all mammals dream? Because all mammals have REM sleep. Since humans are mammals, the biologically correct answer to the question "Why do we dream?" is, "For the same reason that any mammal does, because we have REM sleep." Yet, while technically correct, this answer is not completely satisfactory; for it merely leads to the question, "But then why do all mammals have REM sleep?"
This is a question for evolutionary biology. According to the available evidence, it seems that Active or REM sleep evolved about one hundred and thirty million years ago, when early mammals gave up laying eggs and first began to give birth viviparously (born live, not hatched). Non-REM or Quiet Sleep, on the other hand, seems to have arisen some fifty million years earlier, when the warm-blooded mammals first evolved from their cold-blooded reptilian ancestors.
The evolution of sleep and later of dreaming was far too widespread and behaviorally significant to have occurred by accident, and they presumably came into being through the usual mechanism which Darwin made famous: "natural selection". The idea is that just those genetic variations which provide the organism with some survival advantage are selected by evolution. Due to genetic variability, at any one time there is a wide range of characteristics exhibited in the population of every species. For any given environment, some of these characteristics will be more favorable than others to a species, increasing the probability that those individuals of the species which possess the favorable variation will live long enough to reproduce, passing on their genes to progeny who in turn will be likely to survive long enough to replicate, and so on. If an inherited trait offers a large enough advantage, before long all members of a given species will possess it, and carry the genes to pass it on. Since this must have been the case with sleep and dreaming, we can assume that they serve some adaptive (i.e., useful) function or functions.
All animals cycle once a day through a "circadian" (approximately 24 hours long) rest-activity rhythm. Some animals such as owls and mice rest in daylight and are active at night; others, such as humans, usually act in the light and rest in the dark. Sleep tends to occur during the rest phase of the 24-hour cycles. Thus, one of the primary adaptive advantages or "functions" of sleep is to enforce immobility on the animal during the rest phase of the circadian cycle, both to ensure its resting and keep it safely in its nest, burrow, or home. Mother nature's original idea of sleep (probably also familiar to your own mother) was to keep you off the streets after dark, and out of trouble.
If you recall that NREM sleep arose at the same time that mammals evolved from reptiles, you will have a hint as to an additional function of sleep. Reptiles were dependent upon external energy sources (primarily the sun) to maintain a high enough body temperature to allow them to undertake the business of living (principally feeding, fleeing and sex). Although reptiles enjoy a lifelong free energy subsidy from the sun, it wasn't always at their disposal, for example at night, when they might have an urgent need to escape from some hungry nocturnal predator. Warm-blooded mammals, on the other hand, were no longer completely at the mercy of weather and time of day, because they maintained their own constant internal temperatures. The cost, however, was great: being warm-blooded took much more energy than being cold-blooded. Their inner fires had to be fueled with food that had to be caught at no small energetic cost to the mammal. The need of warm-blooded mammals to economize energy made energy conservation therefore, an adaptive survival trait. To see how effectively sleep accomplishes this function, consider the case of two little mammals with high metabolic rates, the shrew and the bat. The shrew sleeps very little and has a life expectancy of no more than two years; the bat, in contrast, sleeps twenty hours a day and as a result can expect to live as many as eighteen years! If we convert these lifetimes into years awake, the bat is still ahead with three years of active life compared to the shrew's two. There seems no doubt that sleep serves an energy conservation function keeping warm-blooded, fast-moving creatures from burning out too fast. This suggests that there is more truth than fiction to the old aphorism about getting a good night's sleep!
All right, you might say, so that's why we have Quiet Sleep, but why did Active Sleep evolve and with it dreaming? Certainly, there must have been very good reasons for it, since this state has many disadvantages. For one, your brain uses much more energy during dreaming than it does while awake or in Quiet Sleep. For another, there is the fact that the body is paralyzed while you are dreaming, significantly increasing the sleeper's vulnerability. In fact, the amount of dreaming sleep for a given species is directly proportional to the degree of safety from predators; the more dangerous life is, the less a species can afford to dream.
Given these drawbacks, Active Sleep must have offered particularly useful advantages to the mammals of one hundred and thirty million years ago. We can guess one advantage if we remember that this was just the point in evolutionary history when mammalian mothers gave up laying eggs in favor of bearing live young instead. So what advantages might Active Sleep have offered to our ancestral mothers? The answer can be seen, I think, if you recall that egg-hatched lizards or birds break out of their own shells already sufficiently developed to survive on their own if necessary. Viviparous offspring, of which the human baby provides an unexcelled example, are, on the contrary, less developed at birth and often completely, helpless. Viviparous infants have to get through a great deal of learning and development, especially of the brain, in the first few weeks, months, and years of life.
In contrast to the hour and a half an adult spends in REM sleep each night, a new-born baby, who sleeps sixteen to eighteen hours a day is likely to spend 50% of all this time-- as much as nine hours a day--dreaming! The fact that the amount and proportion of REM sleep decreases throughout life suggested to several dream researchers  that REM sleep may play an important role in the development of the infant brain, providing an internal source of intense stimulation which would facilitate the maturation of the infant's nervous system as well as help in preparing the child for the limitless world of stimulation it will soon have to face.
The foremost French sleep researcher, Professor Michel Jouvet, of the University of Lyon, has proposed a similar function for Active Sleep: according to him, dreaming permits the testing and practicing of genetically programmed (i.e., instinctual) behaviors without the consequences of overt motor responses--thanks to the paralysis of this Paradoxical state of sleep. So the next time you see a newborn baby girl or boy smiling in their sleep, don't be surprised if they turn out to be perfecting their perfect smile to charm a heart they are yet to meet!
Well then, so now we know why babies dream. But if that were all there is to it, why wouldn't REM sleep completely disappear by adulthood? Well it might, except that there does seem to be something more to it, providing adults with a good reason to continue to dream. The reason is this: Active Sleep has indeed been found to be intimately involved with learning and memory.
The evidence connecting the dream state with learning and memory is of two kinds: the most direct evidence is an extensive body of research indicating that learning tasks that require significant concentration or the acquisition of unfamiliar skills is followed by increased REM sleep. The second type of evidence is less direct but still quite convincing: many studies have shown that memory for certain types of learning is impaired by subsequent REM deprivation. Psychologists distinguish two varieties of learning: prepared and unprepared learning. Prepared learning is easy and quickly acquired while unprepared learning is difficult and only slowly mastered with great effort. According to Boston psychiatrists Dr. Ramon Greenberg and Dr. Chester Pearlman, it is only unprepared learning that is REM- dependent. In of their experiments, which involved rats, they easily learned that cheese was located behind one of two doors--and an electric shock behind the other: this is called "simple position" learning, and most animals are well equipped for it. If on the other hand, the position of reward and punishment are reversed on successive trials, so that each time shock is to be found where cheese was on the previous trial and vice versa, most animals find it difficult (or impossible) to work out this more complex pattern and learn where to expect what; in other words, for rats, "successive position reversal" is an instance of unprepared learning.
After Greenberg and Pearlman subjected rats to these two varieties of task, they deprived them of REM sleep and then re-tested the rats for learning. They reported that while simple position learning was unimpaired by REM deprivation, successive position reversal was "markedly" impaired. "This finding is noteworthy" they remarked, "because successive position reversal is a task which clearly distinguishes the learning capacities of species with REM sleep (mammals) from those without it (fish)." The implication is that REM sleep makes more complex learning possible than would otherwise be the case.
Greenberg and Pearlman conclude that dreaming sleep "appears in species that show increasing abilities to assimilate unusual information into the nervous system." They suggest that the evolutionary development of the dream state "has made possible the increasingly flexible use of information in the mammalian family. That this process occurs during sleep seems to fit with current thinking about programming and reprogramming of information processing systems. Thus, several authors have pointed out the advantage of a separate mechanism for reprogramming the brain in order to avoid interference with ongoing functions." 
One of these authors is Christopher Evans, whose computer- analogy theory of dreams is presented in his recent book, Landscapes of the Night: How and Why We Dream. The late Dr. Evans was an English psychologist with an abiding interest in computers who proposed that dreaming is the brain-computer's "off-line" time when the mind is assimilating the experiences of the day and at the same time updating its programs.
Not only is dreaming associated with learning and memory, but it also appears to play a somewhat broader role in the processing of information in the nervous system, including coping with traumatic experiences and emotional adjustment. The dream state has also been proposed as a restorative for mental functioning; according to Professor Ernest Hartmann, REM sleep helps us to adapt to our environments by improving our mood, memory, and other cognitive functioning through restoring certain neurochemicals that are depleted in the course of waking mental activity. 
Dreaming sleep has also been shown to play a general role in reducing brain excitability.  It can have a favorable effect on our moods, making us, for example, less irritable. Janet Dallet, in a dissertation, has reviewed a number of theories of dream function, concluding that "contemporary theories tend to focus on the function of environmental mastery, viewed from one of three perspectives: (a) problem solving (b) information processing, or (c) ego consolidation." 
Finally, psychologist Ernest Rossi has attributed to dreams a developmental function:
In dreams we witness something more than mere wishes; we experience dramas reflecting our psychological state and the process of change taking place in it. Dreams are a laboratory for experimenting with changes in our psychic life...This constructive or synthetic approach to dreams can be clearly stated: Dreaming is an endogenous process of psychological growth, change and transformation. 
It might be said of the diverse theories of dream function that they all are partly right and they all are partly wrong: right in so far as they say what a function of dreaming is, and wrong to the extent that they say what the function of dreaming is. The situation is analogous to the traditional tale of the blind men and the elephant. In this story the blind men each sought to discover the nature of an elephant by means of touch alone. From the part they grasped, they believed they knew the nature of the whole. An elephant was like a rope for the blind man who grasped it by the tail; like a rug for the one who grasped its ear; like a pillar for the one who grasped its leg; and so on. In like manner, the proponents of the various theories of dreams have each grasped not the whole as they thought, but a part of the function of dreams. Freud, for example, in surveying the many opinions about dreams, judged almost all of these previous views to have missed the forest for the trees. His own theory, which placed great importance on sex as the basis of all dream content, he considered a "view from the heights," but--as it is perhaps apparent today--Freud himself mistook a wood for the world. And as the irreverent have put it, he seems to have grasped the elephant by the balls. Things could be worse though, for others seem to have grasped the elephant by "the feathers"!
Putting aside, for the moment, the question of the special functions of dreaming, let us ask what is the most basic or general function that dreaming is likely to serve. Since dreaming is an activity of the brain, we must first ask what function brain activity serves? And because the most general biological purpose of living organisms is survival, this must also be the most general biological answer to the purpose of brain activity. The brain fosters survival by regulating the organism's transactions with the world and with itself. These latter transactions would perhaps be best achieved in the dream state, when sensory information from the external world is at its minimum.
As organisms proceed up the evolutionary ladder, new forms of cognition and corresponding actions emerge. The four major varieties of action are reflexive, instinctive, habitual and intentional, in ascending order. Behaviors lower on the evolutionary scale are relatively fixed and automatic, while behaviors higher on the scale are more flexible. Automatic behaviors are best if the situation they are designed for is relatively invariable. So, for example, since we must breathe every minute of our lives, this is very efficiently accomplished by a reflexive mechanism. Likewise, instinctive action is effective as long as the environment we are in is not too different from the one our ancestors lived in. Habit, too, is useful while the environment we have learned to get along in doesn't change too much. Intentional or deliberate action has evolved in order to handle environmental changes ("novelty") that our habitual behavior is inadequate to cope with. This highest level of cognition, which allows intentional action, is usually referred to as reflective consciousness. It is the same cognitive function that we call lucidity, when speaking in the context of dreaming.
Reflective consciousness offers the advantage of flexible and creative action as much to the dream state as to the waking state. More specifically, consciousness allows dreamers to detach themselves from the situation they are in, and reflect on possible alternative modes of action. Lucid dreamers are thus able to act reflectively, instead of merely reflexively. The important thing for lucid dreamers is their freedom from the compulsion of habit; they are capable of deliberate action in accordance with their ideals, and are well able to respond creatively to the dream content. Seen in this light, lucid dreaming does not at all appear as a mere abnormality or meaningless curiosity; rather, it represents a highly adaptive function, the most advanced product of millions of years of biological evolution.
Since the evidence indicates that dreaming serves important biological functions, dreaming cannot be "meaningless biology." On the contrary, dreams are, at very least, meaningful biology. But does this mean that dreams must be meaningful psychology? I think the answer is "not necessarily." If you ask, "What do dreams mean?" the answer will depend upon just exactly what you mean by "meaning." But perhaps we can agree to use "meaning" to mean placing anything--let us say, in this case, a dream--in some explanatory context or other. Please note, however, that explanatory contexts vary widely from person to person. For some, interpretation or translation seems most appropriate under the assumption that dreams are messages to ourselves. Others will seek mechanistic explanations in a physiological or psychological context, and still others will be inclined to treat the dream on its own terms as it relates to itself. Which approach is right? Or, rather, which is right for which dream?
Freud assumed that the events occurring in dreams (lucid or otherwise) were by their very nature necessarily symbolic of unconscious motives. This assumption, although undoubtedly correct in certain circumstances, is equally undoubtedly misleading in others. Many dream interpreters would like to believe that every element of every dream is equally subject to symbolic interpretation, or that "all dreams are equal." This is an understandable belief, for dream interpreters could not expect to stay in business for very long if they were to say of a dream presented for analysis that "this dream is meaningless," or even, "not very interesting." Dreamers meeting with such responses would be inclined to take their dreams elsewhere until they found someone more willing to tell them what their dreams "really" meant. Also, it is a sensible working hypothesis when presented with a dream for interpretation to assume that the dream does have meaning, or, at least, that part of it does.
In the case of psychotherapists and their clients, the relevant kind of meaning assumed and sought is psychological. However, the assumption that every dream contains significant psychological information is yet to be subjected to rigorous test. It seems to me that to assert that every dream is equally informative psychologically or otherwise, informative is like supposing that every sentence you say is equally interesting, coherent, or profound!
There is a contrary way of looking at dreams, the "existential" view, which treats dreams as lived experiences composed of imagined interactions and elements which could be symbolic, or literal, or somewhere in between. Flying, for instance, could be in one case the symbolic expression of any number of unconscious desires, such as the wish to transcend all limitation, or as Freud would suggest, the wish to engage in sexual activity. While in another case, it might be merely the most convenient mode of travel available to the dreamer who wants to move from one place to another within the dream world.
From these foregoing considerations, we would probably be wiser to leave the degree of symbolic significance attributed to a given dream event as an empirical rather than an axiomatic matter--as something to test rather than to assume. It seems safe to conclude that for a given dreamer and dream, flying was apparently symbolic of this or that for a certain dreamer and dream only if such an interpretation either impresses the dreamer as having a sufficiently significant explanatory power for his dream, or if it is otherwise supported by compelling evidence.
It is important to realize that just because a particular dream can sometimes be interpreted in symbolic terms doesn't mean that it was intended as a communication in the first place. If dreams are important messages to ourselves, as suggested by the oft-repeated proverb--"an uninterpreted dream is like an unopened letter", then why do we throw most of them away? This is surely what we do when we forget our dreams and we forget the great majority of them. The "letter- to-yourself" theory of dreams is in even worse trouble when we remember the mammalian origins of dreaming. Consider the family dog: of the tens of thousands of dreams that Fido will dream in his lifetime--how many are likely to be interpreted? By Fido, none at all! By his owners, perhaps a few. But if humans are the only mammals equipped with the linguistic skills to use symbolic language, what purpose could dreaming serve for the thousands of species of non-human dreamers? And if it could serve no purpose to our ancestors, how could it have ever evolved?
I think that the answer is clear. Dreaming must serve purposes other than talking-to-ourselves, as I spoke of earlier in the chapter; moreover, these purposes must be achievable without requiring dreams to be remembered, to say nothing of interpreted. In fact, there is a good reason why remembering dreams might be maladaptive for all non- linguistic species, including our ancestors. To see why, consider how we are able to distinguish memories of events that we dreamed and those that actually occurred. It is something that we have learned to do thanks to language. Remember Piaget's account of the child's development of the concept dream. When, as children, we remembered our earliest dreams, we assumed, at first, that they had "actually" happened just like everything else. After enough repetitions of our parents telling us that some of our experiences were "only dreams" we learned to distinguish memories of inner dream events from memories of external physical world events. But how would we ever have been able to untangle the two realities without the help of other people telling us which was which?
Animals, however, have no way to tell each other how to distinguish dreams from reality. Imagine your favorite cat living on the other side of a tall fence that protects it from a vicious dog. Suppose your cat were to dream that the wicked dog was dead, and replaced by a family of mice. What would happen if the cat were to remember this dream when it awoke? Not knowing it was a dream, it would probably hungrily jump over the fence, expecting to find a meal. But instead, it would find itself a meal--for the dog!
Thus dream recall would seem to be a bad thing for cats, dogs, and all of the rest of the mammalian dreamers except humans. This could explain why dreams are difficult to recall. They may be so, according to this view, because of natural selection. We and our ancestors might have been protected from dangerous confusion by the evolution of mechanisms that made forgetting dreams the normal course of affairs. But if the theory I have proposed for why dreams are difficult to recall is correct, contrary to Crick and Mitchison, remembering dreams should do humans no harm, precisely because they can tell the difference between dreams and waking experiences.
In conclusion, I would suggest that the dream is not so much a communication as a creation. In essence, dreaming is more like world making than like letter writing. And if, as we have seen, an uninterpreted dream isn't like an unopened letter, then what is it like? Having demolished a popular proverb, let us replace it with another, that seems to come closer to doing the dream justice: "an uninterpreted dream is like an uninterpreted poem". If I am right, dreams have much more in common with poems than they do with letters. The word poem is derived from a Greek very meaning to create, and I have already argued that the essence of dreaming is closer to creation than to communication. Are all poems equally worth interpreting? Are all poems equally coherent, effective, or worth reading? If you wrote a dozen poems a night every night of your life, what do you suppose you would find among your several hundred thousand poems. All masterpieces? Not likely. All trash? Not likely either. What you would expect is that among great piles of trivial doggerel, there would be a smaller pile of excellent poems, but no more than a handful or perfect masterpieces. It is the same with your drams, I believe. When you have to do five or six shows every night, many of them are likely to lack inspiration. It is true that you can cultivate your dream life so that the time you spend there will grow more rewarding as the years pass. But why should you expect that every one of your dreams is worth taking the time to interpret? And yet, if a poem or a dream calls out to you to interpret it, by all means find out what it means.
It would be a very unusual poet who created poetry primarily for the amusement and instruction of critics or interpreters. He or she doesn't need a critic on hand in order to be affected, perhaps even transformed, by the poem's creation. When we read a poem, we don't need to interpret it to be deeply moved, edified, inspired, and perhaps even enlightened. Having said that neither poems nor dreams have any need of interpretation doesn't mean that it is never useful. On the contrary, it seems clear that intelligent criticism or interpretation can at times greatly increase the depth of our understanding of a poem and in the best of circumstances, of ourselves as well. It is the same with the dream.
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