The Evolution of Sleep
The evolutionary history of sleep suggests why we need it.
Posted March 2, 2009
We recently experienced a very special day - February 12th, the 100th anniversary of the founding of the NAACP, and the two hundredth anniversary of both Abraham Lincoln's and Charles Darwin's birthday. The NAACP has been one of the foremost organizations in the struggle for civil rights. And we are aware from the many articles and television specials that have appeared recently that Lincoln, of course, ranks at the top among our most significant presidents. Lincoln leads the list of presidential luminaries that include the likes of Washington, Jefferson, Jackson, TR, Wilson, FRD, Truman, JFK and LBJ. And Charles Darwin surely stands as one of the greatest scientists of the modern world- ranking among Copernicus, Bruno, Kepler, Galileo, Bacon, Newton, Maxwell, Boltzmann, Plank, Tsiolkovsky, Einstein, Bohr, Heisenberg, Schrödinger, Gödel, Goddard, Turing, Shannon, von Neumann, Feynman, Sperry, Prigogine, and Berners-Lee.
Despite the revolutionary developments that each of these, and many other scientists and political leaders have brought to the modern and post-modern world, many would argue, convincingly, that none have shaken the foundations of our belief systems and had such far reaching impact on other fields of science, technology and culture as Darwin. As I look across my coffee table I see a young Darwin on the cover of Science, an elderly Darwin on the cover of Scientific American, a celebration of his birthday on the cover of The New York Academy of Sciences Magazine, a cover article on "What Darwin Didn't Know" in the National Geographic and a special issue on Darwin and Psychology 1809 - 2009 on the cover of American Psychologist. With all of this emphasis on Darwin and evolution the question was bound to arise for me - what do we know about the evolution of sleep?
At first glance it would seem that sleep is a bad idea. In most environments animals face the prospect of being consumed by other creatures if not constantly alert to the danger around them. Being unconscious for long periods of time would not seem to offer a selective advantage. And yet most animals seem to sleep in some form. It may be that sleep offers the benefit of conserving energy while focusing on repair of the body, in order to allow an animal to utilize maximum energy while awake for survival purposes. In addition, predator and prey animals generally develop a symbiotic relationship. This is necessary because a predator that developed the ability to hunt 24 hours a day would rapidly deplete all the prey that serve as food. Not only would the prey animal be driven to extinction but so would the predator. Sleep helps even things out.
It was Darwin's great insight that natural selection was the primary factor driving the evolution of organisms. His detailed research ultimately lead to the acceptance by the scientific community of evolution as the explanation for the diversity of life. The best current evidence indicates that all life on earth originated from a common ancestor or ancestoral gene pool. The Earth is believed to have formed about 4.6 billion years ago. Within the first billion years or so of the Earth's history, self-replicating chemical processes began. About 3.8 billion years ago, simple cells developed and by 3 billion years ago photosynthesis had emerged. Two billion years ago more complex cells developed with the appearance of mulitcellular life ocurring about one billion years ago. Around 600 million years ago (MYA) very simple animals first appeared and the early ancestors of insects made the scene around 570 MYA. Truly complex animals, such as trilobites, emerged around 550 MYA. Fish evolved by 500 MYA, land plants by 475 MYA, insects by 400 MYA, and reptiles by 300 MYA. Mammals existed on Earth by 200 MYA and birds by 150 MYA. In a major extinction event, possibly caused, at least in part, by an asteriod impact, the dinosaurs (except for the bird ancestors) were wiped out 65 MYA. Finally, 2.5 MYA the genus homo, which includes modern humans, emerged, with fully modern humans finally entering history 200,000 years ago.
So what about the role that sleep played in evolution? It is generally believed by researchers that sleep evolved as a way to conserve and restore energy. Invertebrates such as insects show periods of rest that are similar to sleep in more complex organisms. Fruit flies have been extensively studied in the sleep field for their well defined circadian rhythms. They show a pattern that appears very much like sleep in that they have periods of about 10 hours per day in which they are at rest. If this rest is prevented they later demonstrate longer periods of rest in an apparent effort to make up for the rest that they lost. This is reminiscent of the need for recovery sleep that ocurs when a person misses a night's sleep. Furthermore, neural activity in the fruit fly during periods of rest is similar to that of slow wave sleep in vertebrates. Since both vertebrates and invertebrates show evidence of sleep, it appears that sleep emerged as an evolutionary mechanism very long ago, more than half a billion years ago. Reptilian animals are thought to be the common ancestors of both birds and mammals. Birds and mammals may have inherited a form of sleep already present in the common ancestor - or they may have evolved sleep independently.
Researchers at the University of Pennslyvania recently documented a sleep-like state in round worms. This state seems to help the round worm with changes in nerve cell synapses and suggests that sleep may be important for brain plasticity. Likewise, when mammals are deprived of sleep they show disruption of synaptic changes that are needed for the brain to grow and change. Reptiles show changes in brain activity that correlates with behavioral changes suggestive of sleep although REM sleep has not been definitively demonstrated. Amphibians and fish seem to have sleep-like periods of reduced activity. Birds and mamals show both deep sleep and REM sleep. Sleep, in part, serves a circadian function in that it is timed to the day/night cycle of the planet. It also involves a homeostatic function in that the longer an animal has been without sleep or the greater the stress it has experienced, the deeper the state of rest becomes. This has been noted even in some insects (such as the fruit fly described above) and is easily observed in humans with the use of polysomnography. While circadian rhythms exist in all plants and animals it is not clear that sleep exists in very primitive animals such as the squid and octopus. As yet there is no known restorative function of a state that could be considered sleep in unicellular organisms.
Recent theories indicate that deep sleep may help decrease synaptic strength which increases during the day as an organism learns from interacting with the environment. Sleep may then preserve memories while preparing the brain to function well another day. REM sleep, which occurs predominantly in the very early morning hours, may function to allow the brain to become metabolically as awake as possible while yet remaining asleep, in order to facilitate waking up in the morning. (Just think about how different you feel when awakened in the middle of the night from deep sleep as compared to waking from a dream in the morning.) If sleep helps conserve and restore energy and helps refresh the brain, the question still remains- why did the strategy of spending long periods of time sleeping evolve? A brain could sleep for short periods of time rather than shutting down for a longer time. The answer is likely that given the vulnerability of being awake all day, animals are actually safer if they sleep all at once, rather than if they break up sleep over a 24 hour period. Of course, animals and humans are able to sleep briefly and maintain wakefulness over long periods of time when necessary, but this does not lead to optimal functioning. It does; however, seem to be an effective adaptation in times of severe stress. Interestingly, birds have developed strategies in which one hemisphere of the brain may sleep while the other remains alert. Whales and seals also show uni-hemispheric sleep. Because they have a different evolutionary lineage from birds, it seems that they thus evolved this ability separately. Uni-hemispheric sleep may have developed as a way of dealing with the danger of predators that can suddenly approach from any direction in the air or in the sea. In humans and mice, decreasing deep sleep may be a coping mechanism for dealing with the danger and stress of having predators nearby. This certainly fits with the experience of insomnia, during which people have lighter sleep due to stress and over-arousal.
It appears that sleep evolved early in the development of multicellular animals and has been preserved ever since. Different animals have evolved different ways of benefiting from sleep given their ecological niche. We can only conclude that sleep is very beneficial to fruit flies, orcas, humming birds, and humans, even if we do not yet fully understand its significance.
Darwin was not the first to consider evolution or the only one to consider natural selection to be its mechanism. But he did carry out what is by far the most extensive research in this area, and organized and published his findings in a way that captured people's interest. Whether individuals were filled with wonder or wanted to run him out of town, they came to see Darwin's name as consonant with the theory of evolution by natural selection. As we study and learn more about the evolution and function of sleep from other, perhaps distantly related cousins, we can grow in understanding about sleep's usefulness and disorders, and chart new treatment approaches along the way.
Getting back to our recent historic celebrations, it's good to remember that in addition to being a stellar scientist, Darwin also opposed slavery and the idea that any animal is "higher" than another. While his theories were later subverted and used to promote racism, like President Lincoln and the NAACP, Darwin helped us to recognize the equality of all human beings. He showed the interconnectedness of life and helped us see that all life is worthy of respect.