The Purpose of Dreams: Exploring the Evolutionary and Developmental Functions

[hypnagogue]

In doing some reading lately I came upon an article that may be enlightening on the subject of the purpose of dreaming, which came up in another thread in this forum recently. The following is an excerpt of that piece, chapter 8 of Stephen LaBerge's book Lucid Dreaming. The full chapter can be found online here.

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 [10] 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." [11]

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. [12]

Dreaming sleep has also been shown to play a general role in reducing brain excitability. [13] 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." [14]

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. [15]


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. [...]

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.

 

[Jeebus]

It probably has to do with keeping the brain mentally prepared.

 

[Moonbear]

Hypnagogue, I'm going to have to delve into this more before I can give a proper answer, but a few things jump out at me as fishy, or at least outdated. I think there's some recent evidence that dreams can occur without REM sleep, but need to verify this. I also don't think it's accurate that babies are dreaming during all that sleep, especially newborns, but how would we ask them anyway?

 

[Moonbear]

Here's one article to start with, while I'm still looking and reading.

Sleep Med. 2003 May;4(3):235-41.
Non-dreamers.
Pagel JF.
OBJECTIVE: Assess incidence and clarify whether diagnostic correlates exist for sleep laboratory patients reporting a lack of dream recall. To awaken, during polysomnographically defined sleep including rapid eye movement (REM) sleep, individuals reporting never having experienced a dream, and determine whether they report dreaming. METHODS: Study # 1 - Incidence and polysomnographic correlates of sleep lab patients responding on questionnaire that they had never experienced dreaming. Study # 2 - Phone interviews with those individuals reporting non-dreaming on questionnaire to reassess incidence. Study # 3 - After reassessment, individuals (non-dreamers - # 16) are awakened during polysomnographic defined sleep (including REM sleep) and queried about dream recall. This group is compared statistically to a group (rare-dreamers - # 12) that reported dreaming as an extremely rare occurrence (mean dream recall latency - 13.5 years). RESULTS: Study # 1: Incidence of questionnaire reported non-dreaming in this sleep laboratory population is 6.5% (N=534) and is associated with the diagnosis of obstructive sleep apnea (specificity 95.6% for respiratory disturbance index >15). Study # 2 - Individuals who report after interview to have never experienced dreaming are more unusual (0.38% of this sleep laboratory population). Study # 3 - None of the non-dreamers (# 16) reported dream recall after waking in the sleep laboratory (36 awakenings in total for this group). This group does not differ, based on polysomnographic, clinical, or demographic variables, from the rare-dreaming group that occasionally reported dreams when awakened (3/12 patients, 3/32 awakenings) - a finding consistent with the reports of previous studies. CONCLUSION: The experience of dreaming may not be as ubiquitous as generally accepted. The group of non-dreamers evaluated in this study reports never having recalled a dream and reports no dreams when awakened during polysomnographicly defined sleep. These individuals might not experience dreaming.

PMID: 14592328 [PubMed - indexed for MEDLINE]

 

[hypnagogue]

Hypnagogue, I'm going to have to delve into this more before I can give a proper answer, but a few things jump out at me as fishy, or at least outdated. I think there's some recent evidence that dreams can occur without REM sleep, but need to verify this. I also don't think it's accurate that babies are dreaming during all that sleep, especially newborns, but how would we ask them anyway?

The article is a bit outdated now that I look (1985 or so). As far as I know, however, the information is more or less accurate.

I'd be very interested to hear about dreaming occurring in the absence of REM sleep-- I've never heard of such a thing. The working assumption seems to be that REM sleep is more or less synonymous with dreaming. It's difficult to ascertain in cases where sleepers report no dreams, but there is always the possibility that they have had dreams and just can't remember them. Probably the best way to solve this issue would be to isolate the neural correlates of dreaming in people who remember their dreams and then look for the presence or absence of these correlates in people who don't remember their dreams.

 

[Rader]

Current Ideas about REM Sleep, Dreams and Dreaming

Current Ideas about REM Sleep, Dreams and Dreaming
http://www.apsa.org/pubinfo/remqa.htm


The Roles of NREM and REM Sleep On Memory Consolidation
http://www.holistic-online.com/Remedies/Sleep/sleep_stages-1-4NREM.htm

Four stages of NREM
http://www.holistic-online.com/Remedies/Sleep/sleep_stages-1-4NREM.htm

New reports
http://www.memory-key.com/NatureofMemory/sleep_news.htm

 

[Moonbear]

I'd be very interested to hear about dreaming occurring in the absence of REM sleep-- I've never heard of such a thing.

Sorry, I'm going to have to keep this short. I am currently swamped with work (this is what I get for taking a few days off to relax over a long holiday weekend). Just taking a quick break right now.

Anyway, I did locate some current articles that show dreams, indeed, can occur in stage 2 non-REM sleep. This was determined by waking subjects in a sleep lab during different stages of sleep and asking them if they recalled dreams. Interestingly, there seem to be some hypotheses being tossed around that the type of dream may vary depending on whether it occurs in REM or NREM sleep. For example, in REM sleep, dreams are more of the type that seem like weird hallucinations, whereas in NREM sleep, dreams are more about ordinary events. This seems to be a reasonably new idea (likely due to the relatively recent confirmation that dreams occur during NREM sleep), so I don't know how well it will hold up.

Remind me to dig up the references. I have them here, somewhere, buried in the clutter of PDFs that has exploded on my desktop in the past few days.

I also happened to like that other article I already mentioned above. I keep swearing to people that I don't dream that often, and now I have scientific proof that I could be right! Then again, they usually reply to my declaration of not dreaming that I am supposed to sleep to dream.