Lucid Dreaming Part 12

The Interpretation of Dreams Revisited

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 the excitatory type transmits impulses to other neurons that cause increased nervous activity, or "excitation," in them. The inhibitory type sends messages to other neurons that cause decreased activity, or "inhibition." The human brain is 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, Freud had studied neurobiology intensively. But in his time, only the process of excitation had been discovered; the process of inhibition was not yet known. Based on the a.s.sumption 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."1 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 at his time, Freud's view of the unconscious mind as a cauldron seething with socially unacceptable impulses and desires appears perfectly reasonable, and likewise, his theory of dreaming can readily be seen to follow from it.

Let us imagine what might have happened if you were somehow able to ask the master himself why you had a particular dream. 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 a.s.sociated 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 supplicants 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 gatekeeper to prevent unruly and unacceptable impulses, memories, and thoughts from disturbing your ego's conscious mind. The gatekeeper, which we psychoa.n.a.lysts 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 that conflict with personal and social standards of behavior are banished from the conscious mind, along with the painful emotions and memories a.s.sociated 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 a.s.sociation, the events of the preceding day dredge up these repressed wishes. Naturally, they seek a way to achieve 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 ruckus 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 'dreamwork' constructed a disguise for your repressed wish, made out of 'acceptable' imagery linked to it by a.s.sociation. Thus, transformed into a superficially presentable image, your wish was able to get by the censor and find expression in your dreams. And that is why," Dr. Freud might well have concluded, "you had that dream, and please note 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. I need hardly add that 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 psychoa.n.a.lysis has strong parallels with Buddhism and other Eastern doctrines. But that brings us no closer to answering the original question, and you might well ask again: "But what did my dream mean? Or was it just nonsense?" In that case, Freud would probably have explained, "Every dream has some hidden meaning; the manifest content of the disguised dream (the dream itself) was the result of the dreamwork'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 a.s.sociated with the original wish, we can uncover the hidden message by reasoning backward from the image through a process of interpretation known as 'free a.s.sociation.' " If you had dreamed, let us say, that you were locking a door, Freud would have asked you, "What is the first thing that comes into your mind in connection with the word '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 s.e.x!

In other words, Freud believed that the function of dreaming was to allow the discharge of repressed impulses in such a way as to preserve sleep, and that the instigating force causing dreams to occur was always an instinctual, unconscious wish. Freud considered these unconscious wishes to be predominantly s.e.xual in nature. In his "Introductory lectures on psychoa.n.a.lysis," he wrote: "Though the number of symbols is large, the number of subjects symbolized is not large. In dreams those pertaining to s.e.xual life are the overwhelming majority. . . . They represent the most primitive ideas and interests imaginable."2 In any case, insofar as the instigating force behind every dream was an unconscious wish-whether s.e.xual or otherwise-it follows from Freudian theory that every dream contained 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 thirty 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 a.s.sume that these wishes play any role in dreams at all, and, even worse for the Freudian concept of meaning, we can no longer automatically a.s.sume that every-or even any-dream has meaning! This is not all the news that 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 s.e.xual arousal, as indicated in males by erections and in females by increased v.a.g.i.n.al blood flow. Had Freud lived to hear of this phenomenon, he would almost certainly have regarded it as a complete vindication of his belief that at the bottom of every, or almost every, dream was s.e.x.

The Activation-Synthesis Model of Dreaming

In 1977, doctors 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, "The Brain and a Dream State Generator: An Activation-Synthesis Hypothesis of the Dream Process,"3 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 model the process mathematically [to] a high degree of accuracy. During the REM periods produced when the dream-state generator is switched on, sensory input and motor output are blocked, and the forebrain (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 toward dreaming as not only automatic and periodic, but apparently metabolically determined; of course, this conception of the energetics of dreaming flatly contradicts the cla.s.sical 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 probably have a simpler neurophysiological explanation: such bizarre features of dreams as the condensation of two or more characters into one, discontinuous scene s.h.i.+fts, and symbol formation may merely 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 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 dreams as "a state-dependent amnesia, since a carefully affected state change, to waking, may produce abundant recall even of highly charged dream material." So if you are rapidly awakened out of REM sleep, you are likely to remember dreams that you otherwise would 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 counterattacks from the psychoa.n.a.lytic establishment, which responded that Freud's neurological models were in no way crucial to his psychological theories. In the view of Morton Reiser, chairman of the department of psychiatry at Yale University and a past president of the American Psychoa.n.a.lytic a.s.sociation,

"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.4

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 researchers, to say nothing of therapists and other 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 experience that, sometimes at least, dreams are much more coherent than Hobson-McCarley's model of "the forebrain making the best of a bad job" would suggest. In my view, 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 dream plots if the higher brain centers were limited to mere improvisation with whatever unrelated props, people, and scenes the "noisy signals" from the brain stem happened 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 so wisely and elegantly constructed that they can and do serve as teaching stories suggests that mental functioning of a higher order 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 producing "partially coherent dream imagery from the relatively noisy signals sent up to it," 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, let us say, to open a door to see what you might find there?

Lucid dream reports abound with examples showing that dreamers can 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 than one to a random brain-stem stimulus. As for intentions and ideas, when dreamers attain lucidity they typically remember what they wanted to do in their next lucid dream; they can also remember ideas and principles of behavior-such as "face your fears," "seek a positive outcome," and "remember your mission." Our oneironauts routinely make use of this last principle when sleeping in the laboratory.

Finally, if all 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 agreements? Of course, the answer to all of these rhetorical questions is that the Hobson-McCarley hypothesis cannot be the whole story. 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 under other conditions, incoherent ravings. And why in some dreams are we deluded and in others lucid? Why are some dreams profoundly meaningful and others pointless nonsense?

As for meaning and nonsense, the Activation-Synthesis model of dreaming seems to completely disregard the possibility that dreams could have any intrinsic or even interesting meaning whatsoever. Given this model of the forebrain struggling with random signals, the most we could reasonably expect would be what computer specialists term "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."5 I can hear it now: A psychiatrist asks a patient, "What does this dream make you think of?" And the patient replies: "An inkblot!"

Among psychophysiologically minded dream researchers, a major criticism of the Activation-Synthesis model was that it was essentially a one-way street, allowing traffic to proceed only 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 as a model, 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 have 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 was "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."6

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 a.n.a.logy 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 s.p.a.ce. These features are to be found in the periodicity and duration constancy of REM and in the stereotyped nature of the activity.7

Elsewhere, he 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.8

Dreaming to Forget?

In a paper published in 1983 in the British journal Nature, n.o.bel laureate Francis Crick (one of the team that cracked the genetic code and unraveled the mystery of DNA) and his 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.9

That, in a nutsh.e.l.l, 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: if these 'parasitic' modes of neuronal activity do in fact exist, they might be "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 "without it we believe that the mammalian cortex could not perform so well."

The mechanism Crick and Mitchison propose, 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 p.r.o.ne to be set 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 relearning exactly what we were trying to unlearn! This would seem to represent at least a partial failure of the 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 a.s.sociations ..." 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 seem to suggest 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 end of all psychological a.n.a.lysis of dreams, all attempts at remembering and interpreting them-in fact, the complete shutdown of the dreamwork industry. Fortunately, it appears there is absolutely no direct evidence for "unlearning" during REM. In fact, there doesn't even appear to be 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, as Crick and Mitchison admit, "A direct test of our postulated reverse learning mechanism seems extremely difficult."10

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 from even a partial failure of the reverse-learning mechanism. Certainly, people who habitually remember their dreams do not seem any more p.r.o.ne to "hallucinations, delusions, and obsessions" than people who habitually forget their dreams. Similarly, if the unlearning theory were true, dream deprivation would interfere with the reverse-learning process, producing disastrous results. However, people have been deprived of REM sleep for many nights-in some cases years- without showing any signs of mental breakdown. So, for any of you dreamers concerned about whether dream recall causes brain damage, I would suggest that there is little reason to worry!

The Functions of Dreaming and the Advantages of Consciousness

Let us return to the question with which we began this chapter: "Why do we dream?" Though we have considered only a few answers here, there are many more that could be, and have been, proposed. But we can justifiably rule out in advance any theory 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 may be, we must dream for the same reasons all mammals have dreamed for more than a hundred million years. So, the question is, 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 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: 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 130 million years ago, when early mammals gave up laying eggs and began to give birth viviparously (offspring born live, not hatched). Non-REM or quiet sleep, on the other hand, seems to have arisen some 50 million years earlier, when 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 thus presumably came about through the mechanism that Darwin made famous: natural selection. The idea is that only those genetic variations which provide the organism with some survival advantage are selected by evolution. Due to genetic variability, a wide range of characteristics is exhibited at any one time in the population of every species. Some of these characteristics, however, will be more favorable than others in a given environment. This increases the probability that individuals with a favorable variation will live long enough to reproduce, pa.s.sing 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 great enough advantage, before long all members of a given species will possess it and carry the genes to pa.s.s it on. Since this must have been the case with sleep and dreaming, we can a.s.sume that they serve some adaptive (i.e., useful) function or functions.

All animals cycle once a day through a circadian (approximately twenty-four-hour-long) rhythm of rest and activity. 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 twenty-four-hour cycle. 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 to 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 mammals evolved from reptiles, you will have a hint about sleep's additional function. 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 (princ.i.p.ally feeding, fleeing, and breeding). 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 the weather and the time of day- 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 were fueled with food, which had to be caught-at no small energy cost to the mammal. The need of warm-blooded mammals to economize energy therefore made energy conservation 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 long as eighteen years! If we convert these lifetimes into waking years, the bat is still ahead, with three years of active life compared to the shrew's two. There seems to be no doubt that sleep serves an energy-conservation function, keeping warm-blooded, fast-moving creatures from burning out too fast. This suggests there is more truth than fiction to the old aphorism about the importance of getting a good night's sleep!