The Evolutionary Basis of Sleep

Tu-Anh Vu's picture

Darwin’s evolution theory revolves around the concept of natural selection.  Natural selection can be seen as a process of elimination.  The quote from Herbert Spencer, “the survival of the fittest” is relevant to the explanation of natural selection.  Offspring that will survive to the next generation are those who possess characteristics or phenotypes that are particularly well adapted to the current environmental conditions.  Therefore the term, “the fittest,” refers to individuals who are capable of coping with the challenges of their environment and in competing with other members of their populations will have the best chance to survive and pass on their genes.  Natural selection eliminates individuals who lack particular attributes that would make them more superior to others are selected against.  Adaptation is what makes individuals superior.  The definition for adaptation is a property that an organism possesses, which could be physiological or behavior traits, of which assists the individual in the struggle for existence.[1]  Some traits that individuals possess will become extinct with time due to the process of natural selection’s elimination of maladaptive traits.             

Can Darwin’s theory of evolution be applied to sleep?  There has to be an evolutionary explanation to explain why man spends one third of his life asleep, or is it a trait that was not eliminated by natural selection because of a mistake?  In Daniel Dennett’s Darwin’s Dangerous Idea, he toys with the idea that sleep might not have a clear biological function.  He says, stating a quote from Raymo:Lab shelves sag beneath volumes of data, yet no one has discerned that sleep has any clear biological function.  Then what evolutionary pressure this curious behavior that forces us to spend a third of our lives unconscious?  Sleeping animals are more vulnerable to predators.  They have less time to search for food, to eat, to find mates, to procreate, to feed their young.  As Victorian parents told their children, sleepy-heads fall behind- in life and evolution.University of Chicago sleep researchers Allen Rechshaffen asks “how could natural selection with its irrevocable logic have ‘permitted’ the animal kingdom to pay the price of sleep for no good reason?” Sleep is so apparently maladaptive that it is hard to understand why some other condition did not evolve to satisfy whatever need it is that sleep satisfies.[2] 

This quote clearly states sleep researches perplexity over the idea that natural selection did not eliminate sleep because of its maladaptive qualities.  Sleeping is not a beneficial trait because it limits the time individuals have to procure food and to reproduce in order to pass their genes to the next generation.  Sleep, a condition of semi-consciousness, can also cause anxiety because sleep reduces the individual’s environmental perceptions and degrades the individual ability to respond to danger.[3]  It is seen as a deleterious act because the realization that the individual is vulnerable to attack while they sleep makes sleeping maladaptive.   In our Evolution class, Hayley Reed agrees with the notion of the maladaptive sleeping trait saying, “if I had it my way I wouldn’t sleep at all… I do think that ‘being up and about, having adventures and completing projects, seeing our friends and learning about the world’ is the whole point of life. I don’t think that the whole point of life is to sleep.”[4]   Reed believes that losing hours to sleep is a waste of time, better spent being productive.  How did a characteristic that is so deleterious to human existence survive after many years of natural selection?  If scientist believe Darwin’s theories of evolution is a working model that explains how species evolve, then through logical reasoning, there must be a reason why Mother Nature has not eliminated the deleterious sleeping trait or else Darwin’s theories are incorrect. 

            Humans spend approximately one-third of their lives sleeping, so there should be a crucial biological reason why nature intended humans to be this way.  Research has been done on rats showing rats that are deprived of sleep die sooner than those that are deprived of food.  If sleep is more necessary than food, having more time to acquire food would not make an individual more adapted, if he loses his sleep.  Sleep can be seen as much a necessity as food.             

Although saying sleep is comparable to the desire of acquiring food is a fascinating research outcome, it still does not answer the question of the evolutionary significance of sleep.  One can not presume that the effects of sleep deficiency of rats are independent of the deprivation technique used or that sleep loss has equally horrible effects in all animals.  Researchers have experimented with the notion of sleep recently and have collected numerous data supporting theories which suggest that sleep saves energy, keeps the individual from being lively at unfortunate times, and that sleep is an adaptation to ecological factors that differ across species.  These findings support the evolutionary basis for sleep.[5]             

Sleep deprivation can have drastic deleterious effects.  The lost of sleep can cause intrusions of sleep into waking that displaces behaviors that have obvious survival value to the individual.  When an individual is deprived of sleep, the missed amount of sleep has to be made up or else the individual cannot perform at high levels for a long period of time.  If this sleep deprivation continues to become long-term, unfortunate effects that are deleterious to the survival of the individual will take place.  Long-term sleep deprivation can lead to skin lesions, hyperthermia leading to hypothermia, increase food intake and sometimes death.  These outcomes were seen on the test rats.  Sleep lost results in the damage and abnormal function of the endocrine and immune systems in humans.  It is necessary for individuals, humans and rats, to get the required amount of sleep.  If the amount of sleep required to function normally are not fulfilled then the individual must have to make up for the lost portion of their sleep, or else they will suffer catastrophic failure in their body to function correctly, thus be less fit to survive the environmental challenges.     

Sleep is evolutionary favored because it is a way for individuals to conserve their energy.  Sleeping for humans is similar to hibernation for animals. Animals hibernate because there is no food (sometimes due to the changing seasons), and by shutting down the brain and body they are able to save energy. With sleep, activities of the individuals are suppressed across part of the twenty-four hour day.  For example: small herbivores and other mammals need to maximize sleep amounts to conserve energy because their relatively high ratio of surface area to body mass makes it costly to maintain their body temperature. Retreating to a warm, protected home to sleep may minimize the energy to keep a constant warm body temperature.[6]  Energy conservation through sleep is particularly important in newborns. Their high ratio of surface area to body mass makes the energy conservation achieved by sleep highly adaptive. Also, animals that are immature at birth benefit from the sleep-induced reduction in exposure to danger.  Newborns are at a greater risk of being eaten, thus they spend most of their time sleeping in a secure hiding space supported by their caretakers.  Though, when a comforting and safe niche is not available, the newborns will cease to sleep many hours.  A good example is of marine newborn mammals, such as infant dolphins and infant killer whales, which sleep little as newborns because the ocean has limited safe hiding places.  But their sleep amounts will increase as they mature.  One can see evolution at work in the ocean.  Natural selection will select newborns that sleep a lot because it limits them to be wary of incoming attacks and ambushes.  The ocean is not a safe haven for young mammals that do not have experience for the predatory world.  Outside from the environmental pressure of the ocean, when body size increases and sensory-motor systems mature in young animals; they will attain greater benefits from active waking activities, which is consistent with the developmental decrease in sleep time.  As the individual develops with time, the amount of hours they sleep will decrease.  Sleep is a beneficial trait because it makes an individual immobile and quiet at fortunate times when it is hazardous to move around and there is nothing else important to do that will enhance the individual’s survival.  Sleeping in the case of a haphazard environment is a necessary trait that most newborns learn or is innate.  The core function of sleep is thus to adapt animals to their specific environmental niche.  When talking about sleep, one can categorize sleep into two sleep states, Synchronized (S-sleep) and Desynchronized (D-sleep).  The two sleeping states have different functions.  The deeper and probably most intensive part of S-sleep, has a physically restorative function to the body, which is more necessary after exercise or when catabolism has been increased.  On the basis of scientific evidence that was discovered recently, the S-sleep phase could be when macromolecules are synthesized, especially in the central nervous system.  These macromolecules are then used in the function of D-sleep.  From the studies of sleep deprivation and the studies of tiredness, it can be concluded that D-sleep may have a restorative function with respect to systems of focus attention (especially the ability to focus on one item while ignoring others); systems involving the ability to maintain an optimistic mood, energy, and self-confidence.  Also, systems involving processes of emotional adaptation to the physical and social environment are dependant upon D-sleep.  From the studies of long, short, and variable sleepers, it can be concluded that sleep, especially D-sleep, is necessary in larger quantities after days of stress, worry, or intense new learning, especially if the learning is in itself somewhat stressful.  D-sleep, thus have a role in consolidating learning or memory, but there is strong evidence that suggest that stress is important and that more D-sleep is needed when there have been emotionally involving changes during the day.  Therefore, individuals that require more sleep are not so much persons who have learned a lot of new facts during the day, but rather persons who have disrupted their usual ways of doing things.[7]  This includes persons who have reprogrammed themselves during their waking hours.  Thus sleep and D-sleep may have a role in consolidating or reconnecting these important alterations made during the day.  D-sleep is able to do this by means of synaptic rebuilding, using macromolecules previously synthesized in S-sleep.  It is for individuals to have both S-sleep and D-sleep for the body to function properly.             

Research has been done on the physiological aspect of sleep, particularly on dreaming.  The discovery that dreaming is associated with a particular kind of sleep (Rapid Eye Movement aka REM) has made it possible for scientists to determine how often and for how long a person dreams and also to observe the effects of differences in personality and daytime experience upon the amount of dreaming sleep a person experiences.  This discovery leads to evidence indicating that there is considerable mental functioning during sleep, thus the brain is still active contradicting the well believed theory that the brain becomes inactive at the onset of sleep.  Evolutionary logic would make one wonder if it would be more efficient and less energy costly to simply shut down the mental apparatus.  But does the presence of an active brain indicate that mental activity is more than an insignificant by-product of a state of sleep, in some sense, as essential to the individual as the beat of its heart.  One extreme hypothesis is that the experience of dreaming may itself be the major function of sleep and that the role of sleep may be merely to allow a state such that dreams may emerge.  Although Sigmund Freud never made such a statement, some have interpreted Freud in this light.  This suggesting leads to the idea for an adaptive function for dreams and it implies that sleep may exists chiefly to allow dreams to occur.  According to Sigmund Freud’s experimental observations, which he postulates that dreaming was a meaningful and motivated activity, on that fulfilled the basic needs of the human being.[8]  Freud felt that dreaming serves functions beyond those directly concerned with the state of sleep.  He assumed that the maintenance of waking personality integrity depends upon the harmless discharge in dream content of tensions or impulses whose appearance in waking life would be most inappropriate or harmful.  For example: it is better to dream that one harms a friend or commits an indecency than really to perform such actions while awake.             

Recent studies employing the rapid eye movement method of dream detection have seemed to some observers to provide a measure of support for Freud’s hypothesis that dreaming is an essential human activity.  The hypothesis has been tested by observations of subjects who are systematically prevented from experiencing REM sleep, and the dreaming associated with it.  Marked deterioration in personality functioning has sometimes been observed as an apparent consequence of this dream deprivation.[9]  Personality in this case refers to more permanent and internalized pre-sleep influences and specifies highly idiosyncratic factors, rather than stable systems of observable traits individuals might share with one another.  Dreaming is necessary for the individual to be at its peak and in order to dream, the individual must sleep, thus stressing the importance of sleep.             

Although sleep may seem like a mistake because natural selection did not eliminate it due to its maladaptive trait, it is a beneficial characteristic that successful individuals acquire through evolution.  The need for sleep is an undoubted fact.  The effects of prolonged sleep deprivation are well known: a lost of efficiency in mental and physical functioning, irritability, and tendencies toward perceptual distortion and ideational confusion.[10]  Experimenters who have deliberately sleep-deprived animals report that the animals may actually die after ten or more sleepless days, there is sometimes an indication in postmortem analyses that such sleep deprivation has produced degeneration in the tissue of the brain and other bodily organs.  There is no question that even shorter deprivation periods seriously impair the individual’s adaptation to his environment.  It’s interesting to learn that deliberate attempts to produce such impairment, for example brainwashing, seem to rely heavily upon sleep deprivation as a technique for breaking down established patterns of thought and behavior.  The biological basis for sleep is that it is a process for animals to be well adapted to their particular niche.  Sleep is used by ocean young mammals to survive the hunters of the sea and for land animals to conserve energy and use their energy in a more efficient way of procuring food.  The process of natural selection did not eliminate sleep because it is a mechanism that organisms use to adapt to a particular environment which makes them better suited to take on the challenges that Mother Nature has in store of them.                

[1] Mayr, Ernst. What Evolution is. New York: Basic Books. 2001. [p. 149][2] Dennett, Daniel. Darwin’s Dangerous Idea. New York: Simon & Schuster. 1995. [p. 339][3] Requadt, Walter. Sleep and Dreams. 2006 [Online:][4] Reed, Hayley. A life of Sleep. Week 5- Evolution/Stories. [Online:] January 2007 [5] John Stear. (Non) Adaptive Function of Sleep. Circadiana: The early bird catches the worm. [Online:] November 2005[6] Requadt, Walter. Sleep and Dreams. 2006 [Online:][7] Hartmann, Ernest. The Functions of Sleep. New Haven: Yale University Press. 1973 [p. 200] [8] Brians, Paul. Sigmund Freud: The Interpretation of Dreams (1900). 1998. [Online:] [9] Hartmann, Ernest. The Functions of Sleep. New Haven: Yale University Press. 1973 [p. 118][10] N. Kleitman. Sleep and Wakefulness. 2nd ed. Chicago: University of Chicago Press. 1963.


Anonymous's picture

Tu Anh Vu did you attend

Tu Anh Vu did you attend Hillsdale School in San Jose California? If you did, I am your first grade teacher.
Where are you now??

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